The magnifier on the WLF should not need a diopter adjustment, nor can I think you can fit one, at least on the WLF that I have.
If you use the prism or chimney they do have diopter or adjustable eyepieces depending on the model. Is the lens maybe in stop down mode? When it is dim, it is harder to focus.

Here is dumb question, is the screen perhaps installed upside down?