Electric Garage heater contactor failure
You're overloading the contactor. 7500 watts at 240 volts is 30+ amps, and the contactor you're using is rated for 22 amps.

You have three choices:


Get a different contactor.
Follow the manufacturer instructions to change the wattage of the heater to 5000 watts or less.
Use the internal thermostat of the heater.