Good answer. I have noted this as well, although a good friend finally pointed out that it was more of my own doing than a lady's failure to respond. So I respond in hopes that others can learn from my mistakes. Not saying this is the issue with the OP or with anyone else, just sharing lessons learned.
In my case, I would contact ladies and write way too much crap (I know, big surprise, right?) I would express an interest, but by the time I got around to actually specifying the date/time, I wouldn't be surprised if she had stopped reading, retired, or died of old age.
Another pitfall I made was to say things like, "I can do X day at X time or Y day at Y time." While I thought it polite and respectful of her time, it apparently left it too vague for many ladies. Worse, it also left me thinking that I had just made myself the proverbial Tebow amongst her client options. Oddly enough, when I now ask for a specific date and time, and it does not work for the lady, many will offer an alternative.
I also have had a few ladies tell me they will NOT schedule via PM or email and only on the phone. Not sure I follow their reasoning on this given they will converse via PM or email, but there are thankfully a lot of things about ladies that I don't understand. For instance, I am still trying to figure out why some ladies that advertise here don't have PM as well but that is probably best for a different thread.