This applies to not just photographers, some models may experience the same problems. I find that if I send a PM to a model I do get a response, I then contact studios, who in turn supply me with available dates and times, I then pass these details on to the model(s). I agree a price if the shoot involves paid work with model, but after that there is no further contact from some models.
Do many photographers/models experience this problem? I spend a lot of time trying to set up studio shoots, only in many cases to be forced to cancel the studio because the model has suddenly stopped replying to confirm the final details, which does not go down well with the studio, which have in many cases turned bookings down because I had booked a provisional date/time for the shoot.
I think its quite natural for some shoots to fall apart in the planning stages (although annoying).
I find the best way to plan is to find out when both parties are usually available?
Another thing I find easier is when people just tell me an exact date - it could be a few weeks in the future - "hey chrissie looking to book you on 23rd of March @x studio for 4x hours of artistic nude starting at 3pm. Please let me know if you are available and if so what your fee for this will be".
I'm not sure if they have booked the studio at that point, or if they have a list of models they will contact for that specific date. I have had to decline a few bookings like tht because I have already been booked but for the majority of the time the photographer and I have been able to sort something out - might be a slightly different time to what he originally stated. I find it much easier than the "I will contact the studio/contact you back". Generally though when people don't set dates to me then I will ask when their usual availability is. I will then send them a list of the dates I have free in the next 6 or so weeks.
Maybe try just telling the models when you are needing them, then if they are free they are free if not then you move on to somebody who is free.