You need at least 2V of "head room" for the 7812 meaning that it wants at least 14V coming in. If you don't have a heat sink on the 7812, then it's probably getting pretty hot and possibly shutting down. You should add a .1uF cap on the regulator output in parallel with the cap you already have.
If it were me, I'd be tempted to see if a 1K (or so) resistor across the 12V supply output would load the 17V down to ~12V and skip using the 7812 altogether; either that or get a better regulated 12V supply.