Be sure the hole is as deep as possible. Roll form taps are very helpful also in gummy materials like copper. Do not try to go back in the hole unless you have rigid tapping. Drilling is crucial. If you're not using the new style drills, like the square drills with no web, your hole must be spotted.
Any drill walk is deadly to any kind of tapping.
Spotting drills are fine, you don't need a centerdrill unless the surface is uneven. I have many years of experience and would be glad to discuss the specifics of your problem. I'm the Tech.