space in UNDO tablespace not being released

Hello, I have a problem that space in the UNDO tablespaces is not released. We have had a situation, when the undo tablespace was used for 100% and the space was not released. We dont use hard retention time on the tablespaces and they are not autoincrement enabled. I think about possible problem with autotune retention time, which is very log, because we dont have usually so much load on the system. The undo is usually bello 85%, which is normal target for schrinks i guess.
Is there any relevant docu available describing the behavior in detail? And how can i get rid of the old undo segments? The only way i know is to drop the undo tablespace and create a new one. How can i analyse the situation when the undo gets 100% full? how to see and check the data there?