I can only use two knobs to make the igniters work on the 5 burnners of the cooktop. If I need to replace the knobs, how do I get to them? Can anyone help?

THey are under the cooktop. How hard that may be depends on which specific model cooktop it is.

If its like the one I have (with the black glass top)... that might be impossible because the nuts that are on the gas tube to the bottom half of the burner assy HAS to come off... ALL of them and they tend to corrode and seize tight. I've replaced the cooktop once for that reason, but the next time I'm buying a different brand since I'm not doing it a third time.

The knobs lift right up by pulling up... the problem exists however in the gas valve assembly. THe knobs are only decerative plastic