Z360W strange behavior with Cokin filters

[enrico_pocopagni1]

Hello all; I noticed that my Z360W with circular screw-in filter

(like Mamiya polarizer) works fine. On the contrary, when I slide

Cokin P filters (like warm or graduated ND)in their mount, the image

in the finder becomes fuzzy and focusing almost impossible. Lens is

pre-focused and locked, sliding filter is carefully performed,

filters seems to me perfectly flat. Will final image be fuzzy as

well?, to what physics is this behavior due?, how can I prevent

this. All my other lenses (50 ULD, 90 and 180W-N) behave the normal

way, either with screw-in and Cokin filters. Many thanks.