OK, I'll try to figure out what's wrong...let me know if I go off base...
When you first apply pressure to the keg you released the pressure to purge air out of it, correct?
What pressure setting did you have on the gage, 12?
Then you poured a beer and it dropped to 2, right?
Then you should have put the gage back up to 12.
Incidentally, at what point did you try to carbonate the beer? What psi, temperature and time period did you use?
There are many, many ways this could have been done, but most commonly is either the "shaking" method or the "set it and forget it" method.
The "shaking" method is just that. First you purge the air out then set the gage for anywhere between 20 and 40 spi then start shaking the bejesus out of it to get the CO2 absorbed by the liquid. I prefer to roll mine on my lap or on the table/counter top...inlet side down. You use more CO2 when the beer is warmer/room temp than if it's already cold.
With the "set it and forget it" method you place the keg in the fridge to get (and keep) cold and set the gage for 12 psi for about a week.
Have you figured out where you went wrong yet?