Originally posted by calsan I worked out "user error" was causing the blank shots, so it will just need an adjustment for the shutter speed.
That is a pretty camera, a beaut of a lens and your case looks pristine.
Late 'S' body and the K body Pentaxes can suffer from a dragging 2nd curtain at 1/1000, which might cause your uneven exposure. I just bought an ESII (Silver) knowing it has this fault. I plan to get a CLA immediately - you might consider having your KX evaluated for this.
Did I mention KX is my favorite film camera?