ManMuchwano93z28

I been driving my car lately. and I notice that its not catching overdrive and sometimes not even 3rd.the only time it will catch the next gear is if I punch it and it will shift at like 5000 rpm. I first thought it just needed some fluid but it had some in there. The car does have a b&m shift kit also. was thinking if that was the problem. Has this happened to anyone before..?? Please help.. I'm new to transmissions