That info says a lot: if your film does not lay flat in your carrier, then you will have this problem.
Question: is there a curl in your film ?
Try to flatten that first.
Check allso if the neg's are placed propperly.
You can check with the neg carier out of the enlarger with a neg strip in it.
Your enlarger should have some kind of provision for easy removal of the neg carier, after the carier has been replaced the head should come down and hold the carier firmly into place.