What enlarger and what size negative?

First focus the negative, then don't touch anything and watch thru the grain focuser. Is the image going out of focus? If so, you need to adjust/tighten the focusing controls or lens platform, depending on which enlarger you have.

Then, I would check the alignment of the enlarger; negative stage to baseboard, lens stage to baseboard. Actually I would do this anyway, so you know the enlarger is in alignment.

After the enlarger checks out, and you still have that problem, then I would look at the negative carrier. If you have a negative buckling problem, you may have to resort to using a glass carrier to hold the negative FLAT.