Issues with my enlarger?

I have a Beseler enlarger that I have used for several years without problems. I have a 50mm El Nikkor lens for 35mm film and an 80mm El Nikkor lens for 6x7 film.

Today I wanted to print an 11x14 from a 35mm frame. I set everything up and found that my 50mm lens couldn't get the image to focus at 11x14. I twist the focus knob and the lens moves up and the image gets sharper until… It stops moving because it runs out of track and the image still isn't clear. Even setting to f/16 won't make it clear enough to get a decent print.

I did some tests and it turns out I can't even get it to focus when printing an 8x10. The largest image size I can use before the lens will not achieve proper focus is about 5x7.

Has anyone ever experienced a problem like this before?

Could be the lens, as Alan states.

It could also be the condenser bellows have moved. Have you checked that? The bellows would be completely compressed for the 35mm set up. It maybe the bellows have gotten old and are sagging or a nut/bolt has come loose and the bellows are no longer in the correct position.

Also check the alignment of the enlarger. If it got bumped, even slightly, it may no longer be aligned properly. You can buy alignment negs which you mount into the enlarger.

If in the past you were able to make 8x10's and 11x14's and focus, likely a mechanical change has taken place. First check the 50mm enlarger lens and its lens board. Likely the lens board is not a flat board but of the recessed type. This places the lens closer to the negative. Is it possible you dismounted the 50mm for cleaning and then failed to reinstall correctly. Anyway the problem is that the focusing mechanism is failing to place the lens close enough to the negative. Recheck the negative stage for correct position. Check the lens board receiver mount for correctness. Look over everything. You need to position the lens so it ends up a few millimeters closer to the negative.