why does the nanny not mostra Fran's dad in some of the ones he is in it dosent mostra his face

Fran's dad, Morty, was played (when te could fully see him) da Steve Lawrence - big time singer who te could often see on the Carol Burnett show. I would presume that since he was probably not as available (or affordable) as the script called for him to be, keeping him unseen was a good way around that.

Well I always thought it was kind of a running joke, they always talk about him but never see him. They do that on a lot of sitcoms, they give really vivid descriptions about a character who is not major but not minor frequently, to leave the viewer curious wanting more. I've seen it on home Improvement, Hannah Montana, and I'm sure many other shows too.

the want to keep it a mystery so people can imagine what he looks like

I imagine it was a running joke to have him absent in the early and middle seasons, but I believe he was then cast for the last two o so seasons.