A few things to try and help out:
Another maybe not so obvious one - the key needs to be in the "on" position with the engine off.
You will know you are in re-learn mode because the turn signal at the corner the car is looking for will illuminate. The DIC will probably also display a message. If you don't have that going on, don't even bother trying to go any further. It won't work.
There is also a TSB out there for some cars that the TPMS system had bad software, and the ability to detect sensors by changing pressure was not enabled at the factory. The only way to fix that is have the dealer re-flash it.
If you have a cheap generic sensor, they sometimes need to be "activated" by a much more expensive tool then the one I mentioned. Basically they have to be programmed to your frequency and car. I would think any tire shop should be able to do this.
If you do think you have one of the bad TPMS systems and don't want to pay the dealer to fix it, I believe the tool I mentioned elsewhere will still work, assuming your sensors are active and you have managed to enter re-learn mode. Or again, any decent tire shop should be able to do it.
2007 Solstice GXP
#3 ASP, DET and NWOR SCCA
"It's a Kappa - not a classic"