I haven't been on SB, but Ashley Madison is very similar to what you've described. Inquiries that you have to spend credits to respond to... then crickets.  I've come to believe it's orchestrated to make you use your credits and buy more.  And I also believe many (not all) of the profiles are bogus... written by some girl in China or the PI.  One of the dead giveaways is when they list their weight in kilograms. All of that being said, there are some profiles that are legitimate, but as you said, the ladies are just so-so.  You won't be shouting yippie-kai-yay.

That is excatly my impression of SB and SD (same thing, really). I get 5 new messages a day there, but none of them ever respond if I open the DM and send a note back.  The whole thing strikes me as a fraud.  I never get new messages on SA, but I send out notes and get responses.  

It is very easy to burn through the dollars on SB/SD.

is that when gals sign up the default is they send a message out automatically (not sure exactly but maybe to your local area or if you vew the profile.)
I haven't used it so not sure, but if they seem to be boilerplater messages without much in them, they may be computer-generated and not a real message. If the gal favorites you it would be different.

I  assume we are talking here about  Secret Benefits -- SB.  Both babes and daddies are going to have different strategies on SB than on SA (Seeking.)   On SA daddies can blast messages to anyone and everyone, the race is on to make contact before your month is up.  Babes get blasted by any and all comers.  If the babes initiate contact it is because they are targeting something specific.  (Aside from the foreign ones vying for a green card.)
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But on SB it costs guys to send a message.  Here the babes have a lot more incentive to broadcast introductions. They aren't going to get the same barrage of interest from guys who have to have discipline in who they contact.
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My strategy on SB is to only respond to introductions and not cold call.  On SA the best strategy is to broadcast to them all.    
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I've never got the impression there were bots on SB.  Most of them look like regular girls.  The few 10's or model material may or may not be real, but I'm guessing want top dollar.  If I want a model or a 10 I'll go with a pro.  They actually have more realistic price expectations.
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I'm not a high volume guy but I've been BCD with nine babes off SB -- spanning about three years.

Since I had trouble with my SA profile I decided to give SB another look.  Browsing the profiles in my area I saw a girl I dated off SA a few years ago and would love to date again.  That was the final thing that made me decide to purchase SB credits.  Since I've been on only about a week I've seen several others who I also saw on SA.  One girl I had a coffee date with last year, one girl I actually had BCD a month ago.  So I know for certain that at least some of the SB profiles are legit.  But everything on the site costs credits.  Do you want to send a message to someone?  10 credits.  Want to know if they read it?  10 more credits.  It's really easy to burn through those credits!  So I agree with the differences in strategies on SB vs SA.  I won't waste credits sending a DM to someone first.  If she sends me a DM, and if she's attractive to me, only then will I spend the credits to read her message and reply.  So far I have not received any responses.  Sure seems scammy to me.

An idea I overlooked until actually this morning on SB was that once you pay the credits to contact the first time, you can message them as often as you want.
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And that's the tip.  Because if their inbox is like ours, their messages at the top are most recent. So what happens when they get a ton of messages.  Your message is going to scroll down farther and farther and they may get exhausted replying to others before they go too deep in the inbox.
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Therefore the tip on not getting any replies is to keep pinging them.  It's the only way to get your message to the top of their queue.

I too was banished from SA only to end up on SB. I get the feeling sometimes they are trying to get you to burn your credits. OTOH I've hooked up with roughly 10 ladies at far lower rates than I see here.

SA seems to be adrift in terms of its mission. A prior poster explained why. It used to be a great place to create SB/SD arrangements, now it's all over the map on who it attracts. I found my most recent SB there, though it has been way more difficult to find someone over the last year, So I decided to shift to SB after my SA membership ran out. After I got the hang of how not to fly through credits I have now done meet and greets with a couple of women. I have another one tonight.  I daily DMs. If they haven't viewed my profile I ignore them. If their profile fits my demographic and I like what they write and the pics are pleasing I selectively write back to them. Like others have said proactively reaching out to ladies has been a 1 in a 100 chance of getting a reply, but then I don't follow up if I hear nothing as I usually move on, though I did like the posters suggestion to keep pinging them as it doesn't cost anything. For those I have met it's like SA in that they may not make the grade. However the quality is there. They don't appear scammy, no catfishing so far, and they are not asking for outrageous amounts (unless they are very hot looking and they know it.) So at this point I would say SB beats SA. I like the idea that once I have found1-2 SBs (2 is about all I can handle ongoing), I can pause and not use credits which will be great when these SBs run their course.