There can be a few reasons for client disconnection:
1. Devices loosing connection while roaming due to lack of coverage overlap between APs Service area.
2. Frequent Channel changes causing devices to loose connectivity as there is heavy interference.
3. Devices going to sleep mode.
4. Network drivers not updated could cause the issue.
I would ask if the issue is happening only with one device or multiple. Also, are these Mac or Ios devices?
If its happening with multiple devices then you have to check following changes:
1. APs have overlapping coverage.
2. Assign static Channel.
3. Ensure that the drivers are updated and have stable OS version.
Rahul's suggestions are the first things I would check. In addition, have you done any command line tuning like enabling Smart Roam? Overly aggressive smart-roam settings can also lead to unexpected wifi drops.