I think Grace pretty much covered the speculation part but I can also add that there have been a few updates on the site & when logging in, it mentions something about some requests being sent to the provider email, rather than the actual inbox. So if said individuals do not check or look for requests in their email, they may not be aware of your requests.
Of course it's easy for hobbyists here in the Dallas area to just send a pm thru Eccie, but for those that travel or in other Parts of the country where the boards are not as active, P411 actually helps a lot in screening & scheduling an appointment.
Reviews also help a bit, but a pm to the provider or providers you would like to schedule with, after a screening request probably wouldn't hurt either.