Could be many causes.
Grounds
Plugs
Switch
Timer/relay
Trouble shooting sequence would be to determine if 12 volts is getting to the plug buss bar.
Then, if so, determine if plugs get warm/hot. (should be warm to fingers)
For individual plug test, disconnect the buss bar and resistance on each should be very low ohms (like 10 ohms ea or less) Ohms = OK
Ignition switch is another very likely suspect, check volts in/out.
Then comes the plug relay/timer, Power in/out?
If all else fails, (using a jumper from battery pos) 12 volts directly to the plug buss for 5-10 seconds should heat them up for a good start.