Why TM runs 2 separate pre-paid operations (the other being MetroPCS) is beyond me.
Prepay customers of "regular" prepay operations know they are paying less -- therefore don't expect or receive all the frills "regular" customers of the 4 major carriers get. TM prepay, from posts I read here and elsewhere, have almost always used overseas CS. It appears you can get domestic CS via Facebook, Twiiter. (I don't even have a Twitter account and enter FB once a month - so can't comment.
That says you're only allowed two unlocks a year. You're not specific as to whether you're trying to unlock on the same account, or through different accounts, but if it's the same account, that could be the reason. Also, there's a number of days the device has to be on T-Mobile (not sure if it means the account requesting the unlock or what).