I would say raise your rates for PM. But, being you're on CB, I think it's a per entrance into the room isn't it? Meaning, if I tipped for PM, then left for an errand and came back I have to tip again to PM. If that's the case, then definitely raise your rates. But, I can't say to what would be a fair rate for it yet be discouraging enough for those who are looking for cheap PM with you.
As a member (I'm on MFC), I usually do tip for Friend add which gives PM capability with models I visit frequently. But, I always try to keep PMs to a minimum and try to keep the focus in the room. Rarely do I expect a timely response to a PM. But, if was and it's been a while, I'll send a tip note asking her to check my PM.
Setting expectations is a good thing when it comes to PM's, I've seen where models have verbally stated they prefer communication in the chat room as well as have a bot with the same message.
Hopefully you can get it sorted out, and allows you to focus on the room more without loosing any members.