I discovered the other day that the Pentax DA* 55mm lens has very tight tolerances for the provided lens cap. It fits tightly over the outside of the lens and doesn’t allow any overlap by an attached filter.
I was trying to use a B+W MRC Nano UV-Haze filter which slightly overlaps the outside of the lens.
A close up photo shows the 1mm overlap.
That extra 1mm prevents the lens cap from attaching. I’ll try another brand soon and post which one fits.