Hi Twee,
It could be a couple of things:
A technical error with the system not registering the facility
T-Mobile haven't put you on the correct Tariff (if it's new). This happens a lot.
Your bill might not have been paid. Most likely if set up to pay by Direct debit, this has not been set up correctly, deleted by mistake or there is a problem with your bank account.
Worth quickly checking your payments have been made and there are no problems with your bank (just in case).
HTH
Sparky.