Account in Server App

We use Mac Pro server with Lion 10.7.4 OS, and we use Server.App to setup around 100 account, and all acount in Workgroup. we found:

if lots of people log in iMac in the same time, someone may log in fail, should try again.

also if they lock screen long time, since they re-log in system, may fail, then we should try again or re-start iMac.

also we found some application can't pup-up new window sometimes, if we log in with this server account. but local account no this issue

Please support me, thank you