If you make your own burning cards, make sure they are nice and thick and have shaped holes that you like to use for that area of the print.
I like my cards with a white surface on top so I can see the projected negative and use that to aim the burning opening to that area precisely.
To get a over all even burn you have to work the burn in a pattern and use the same speed when going over that pattern. Right to left, then back left to right, then up and down, down then up. Until you have cross hatched the area you need to burn. Obviously smaller burns don't require this much diligence, but going over a larger swath of space like sky does.
You can also try split grade printing to bring out details you have been trying to bring out with burning. It may help with decreasing the amount you need to burn in.