Sometimes the oven has not cycled enough times to get the temperature even. My repairman says let it cycle....turn off and on three times....before trying to bake. Seems like a waste of energy to me, but it seems to work too.
I recently bought three large cookie sheets from Copper Chef. The baking surface is a patterned basket weave. They bake wonderfully where all the others were having problems. Now that I've got cookie baking figured out, I'm trying to come down with diabetes and cookies are way down the list of things to eat.