I've already logged this with SDL Support, but wanted to bring it up here.
Basically, Trados SP2 has a new *feature* which disables locking/unlocking segments when Track Changes are enabled.
I believe before you would get an error message when you tried locking a segment that included Track Changes,
so perhaps the developers thought this was an improvement to prevent errors.
However, if you need to lock/unlock segments and also make Track Changes in the same file,
you have to turn Track Changes ON/OFF multiple times which is tedious.
A better solution would be to disable locking segments only when there are track changes in the segment regardless of whether Track Changes are ON or OFF.
Anyways, my two cents.