I think with most ladies (with PM's) if you are specific and requesting an appointment on such a date etc then you'll get a reply.

I find "enquiry" PM's then to get an average if non-response on the basis of well just questions/queries on the punters part to the escort that she doesn't have the time to respond (unless being a regular)

...same really with phone calls...you have to be definite in what/when/where you want otherwise (i think) you might be wasting their time (even if you are in all purposes genuine)