I've bought a CPL lens 77mm and a 77-55 step down adapter, but the problem is the step-down adapter is not fitting on the lens. The CPL fits fine on the step down but the step-down is not fitting on the lens, could some one point out what I'm missing?
If you are adapting 55mm filter threads on the lens to a 77mm filter you need a step UP ring; 55mm male to 77mm female.
If you bought a step DOWN ring it will have 77mm male threads and would fit onto the front of the filter; but the 55mm threads will be female and unable to mount onto the lens... this should be fairly obvious visually. You could adapt it with an additional 55mm reversing ring (55mm male-male)...
If you actually do have a 55-77mm step up ring and it won't mount; I would guess the threads or roundness are slightly off. Cheap aluminum rings are notorious for this, and for getting jammed on the lens/filter due to poor tolerances/machining... quality brass rings are far better.