Create a few more rb segments,since you have enough space i guess you have more transactions than the rbs might consider handling..
alternatively query V$waitstat to query if there are any waits particularly in undo header, undo block etc..

or restarting the instance will also reset the dba_rollback_segs parameters.......

Check also you optimal clause, depending upon the shrinks,and aveshrinks parameters, you need to adjust them.
if shrinks are low and aveshrink high then the setting is good.
If aveshrink is low and closer to the current setting then it's fine if not the optimal is too high.