Why do all the plates on my cooktop not burn equally hot

Was this always the case? The control or the burner itself may have a problem, and it usually isn't something that can be handled by a homeowner. Why not call a shop and get some options? They're the experts. Have the make, model and serial number of your cooktop (and any other information off the manufacturer's tag) on a piece of paper and then call a shop. Ask them about what may cause the burners to run at different levels and see what they say. You'll be "fishing" a bit for information, and you can call around to see if you can get it. They can at least give you a price to make a service call, and may give you some indication of the problem (which is why you're calling. Fixing a cooktop isn't quite like changing a light bulb, so don't get too cute with this one. Be smart about it. It may come down to having a technician drop by. Call around before selecting a shop to do the work to see which one you can "develop a rapport" with. Best of luck with this one.