In the setup you have with your panel, each circuit is clasified as a main. You can legaly have up to six mains.
I would agree with an earlier poster as to a neutral issue. As for testing the incoming power with the meter, one lead would go on the neutral bus and the other would be on one of the screw terminals where the larger black wires enter the bottom of the breaker panel. You should have ~240 volts between the two terminals at the bottom of the breaker panel and ~120 volts from each to the neutral bus. As suggested, test this both with and without the heat lamps connected. I expect you will find that you have less than 120 volts to the neutral bus and the voltage will lower with the heat lamps connected. If I am correct, your issue is ahead of the breaker panel and an electrician will be required.