Originally Posted by RalphLambrecht
It's a combination of two things. Some paper's sensitivity to UV and some lenses inability to focus UV at the same plane as other colors in the visible spectrum (which we use to focus).
Ctein elaborates on this in his "Post Exposure."
This was more of an issue in the past, and one of the reasons that blue filters came with some grain-focusing devices; by focusing on with blue, you'd be closer to the focus of the UV.
Nowadays this is rarely an issue: most lenses are better corrected and most papers are no longer so sensitive to UV. Plus, if one stops down a bit, which seems to be the practice more now than in the past, since papers are faster, you get a considerable depth of focus at the enlarging easel which alleviates this effect.
FWIW, I don't think the OP's problem has anything to do with chemical focus. Likely just difficulty hand-holding.