I have not experienced this. I usually reach out to ladies on P411 with my initial contact. But even when they are not P411 members and I send emails, I usually get a pretty quick reply.

With Respect to Emails,  In general less is more.  

Name date times  ( have multiple times and dates )  
references  1 2 3
P411 and handles

Leave out wants and desires or information about your dog

Do you check in your spam folder? I use protonmail and many clients have told me that my emails go into their spam.
Are you inquiring about services and/or rates? I do not answer those type of emails and some other providers will not either. I would also suggest resending your email if you do not get an answer your email might have went into her spam folder.

I noticed now that ladies seem to prefer text over email. As far as reaching out in general, I get a response about 80% of the time whether it be via direct email or P411(which I no longer have btw). Since you’re reaching out to ladies who are established, they likely prioritize their regulars over new clients.