Is it normal that i cant have an orgasm unless i concentrate?

Ok so when i do have an orgasm, it's great. Probably just as good as anyone else's. But the thing is if i don't try hard and concentrate and tighten up my leg muscles, i won't have one.
No matter what position, even if im getting oral, i can't just relax and bust one..i really have to try. does this happen to any other female? please tell me im normal