I am relatively new to Kerbal, but have a very simple rocket with two liquid engines surrounded by solid (thumper) boosters. Trying to establish orbit, but after I have shutdown the liquid motors (once the AP is sufficiently high to coast), when I try to restart the motors at AP to establish initial orbit, the indicators show that the motor is restarted, but the motor doesn't burn (no thrust). The throttle is fully open, the motor just doesn't start. It seems to be somewhat intermittent as the first liquid stage starts right up no problem after the boosters burn out.
Am I missing something fundamental that is required to restart the engine?
Thanks.