Thank you for your question.
I can understand wanting the main burners to light like the side burner and back rotisserie burner.
As the grill ages the inside area on the grill begins to develop rust in the areas where the electrode need to arc against.
The front panel where the igniters mount will also get rusty and will not carry the ground for the igniters.
The rear burner and the side burners are usually not used as much.
This model has two igniters shared over 4 burners with flash tubes to light the burners.
Since you say that the back and side burner work then it either has to be wiring from the spark module or air gap on the igniter contacts.
If you have replaced the igniters and you have the igniters gapped to about the thickness of a nickel then it is either wiring to the igniters or a rusty panel mounting.
If you are not getting the igniters to spark I would remove the grates and examine the igniter boxes for rust and connection on the front frame.
I would first check the wire harness and that the igniters have a good connection.
The wire harness is: P2637A.
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