This may be a bit low-tech, and you will definitely not want any witnesses but...
try resting the back of your head on the baseboard of the enlarger. Look up into the lens with the enlarger lamp on and voila, you will see the neg, carrier, etc in sharp relief. Any funky edges of, say, the neg carrier will be immediately apparent. Remember that if the left side of your print is bad, then the right side of the neg will show the problem, if indeed the problem is up inside the enlarger.

Hope this helps.

Thanks, Jon.