If you call them, do you get an answer? Because some providers may just be MIA, for one reason or another, and it has absolutely nothing to do with you.
If you want to be sure to get replies, or at least minimize the number or no replies, try this.
Start with providers who are posting on your local board. They are looking for business, and waiting to hear from you. Then check their web site or ad posts and see how they want you to contact them, and follow their rules. Make sure you qualify - if they require two references and you provide nothing, they probably will not even reply. I spent a month trying to contact a lady I had just seen a few weeks before and was trying to figure out how the hell I had pissed her off, until I went to her web site, checked the contact me page, and discovered she had changed phone numbers (she wanted text messages). Not so amazing, she replied immediately when I used the right number. In another case, I emailed a lady who requires references, but I had none. This was the one time I actually sent a second email - when I reread her web site and saw what I had done, I sent an apology and asked her to reply so I was sure I had the right email for when I had references. She replied and I did get screened later, though our schedules have not meshed yet.
Look for providers with current reviews, as you can tell they are working now. Finally, if you are looking at their ads, look for ads posted in the last 30 days.
Even if you do all that, you will still get no replies, as Phil noted. Some ladies are, well, disorganized, or having too much fun, otherwise engaged, or you may just not seem right to them. I recently had two separate experiences with ladies who I emailed, as instructed and got very quick replies - yup, you are screened, here is my availability, let me know. I answered right back with a date and time, asking for confirmation. One replied 5 days later, three days after my requested date and time, asking if we were still going to get together (no, I replied, I left town two days ago), while the other replied 6 days later, the night before we were supposed to meet, sort of confirming the time, but switching from incall to outcall and since I had just confirmed with another provider I had to say, so sorry, made other plans, which pissed the hell out of her, but c'est la vie. Won't ever get together, I guess.
Anyway, that seems to be normal. I also agree with the 48 hour rule - if they do not reply, or confirm a date within 48 hours, just move on because there is no guarantee when or if they will reply. But do not get mad - they are just folks with lives, and shit happens even to the most organized and professional of people.
The one thing I hardly ever do is send a second email. Just move on if you do not get a reply. Maybe that would not work as well if I lived in a smaller city, but I do not, so there are lots of options, too many options.
Zig