Time is just the easiest to rule out of a list of MANY things... check the logs via Utilities > Console on both server and client and copy and paste...
...and how many op-hours are on each... and when did they last have optics cleaned? First thing I'd be suspecting is dust buildup on internal mirrors...
If the tablets run Android 4.2 or above you'll need to sideload Flash onto them as it's been pulled from the Play Store since 4.2's release, and it's...
