^^agreed. sounds like on the 1st tank you let the motor cool down with the piston at top dead center. not supposed to do that. the pistion to cylinder "pinch" is tightest at TDC. when you let the motor cool down with the piston there, it can get stuck.
use your after run oil applied through the glow plug whole, and use a hair dryer to heat up the motor. that should allow the piston to move again.