Few things to help us on the forums who volunteer to help you:
1. Mind posting your config? (and please remember to strip out External IP addresses as needed for security reasons)
2. This is an RB951 - how many users are connecting and running through the system?
I am wondering if it is a limit due to hardware you may be hitting the upper limits on how many users can pass into and through it.
Glenn Kelley | MCTNA, MTCWE, MTCTCE, RHCE, RHCSS
USA Based 24x7x365 Mikrotik, Juniper, Ubiquiti TAC & WISP / ISP Blind Label Support Call Center