This usually happens when the gimbal control circuit thinks it encountered physical barrier which doesn't allow it to fully level.
Make sure all the arms move without any resistance. Even a slightest resistance will cause the gimbal to give up.
Check if any of the arms motors gets hot.
Make sure the gimbal does not give unusual sounds when working.
You may also check gimbal status in DAT logs; there is probably information about what went wrong, but I'm not sure whether the tools we have are able to read it (DatCon is the simplest to use).