You should open a TAC case in parallel to this post because there could be a number of reasons why this would happen. We can ask things like: what version of ClearPass, Are you using controllers, or instant APs and what OS is this happening to, but it would be quicker for TAC to get your actual data to diagnose...
With that being said:
- What version of ClearPass?
- Are you using a controller or instant-based APS?
- What version are they running?
- Do you have more problems with specific client operating systems than others?