Talk to Urologist on Erectile Dysfunction

i am a physically challenged person. i have polio in one leg (right leg). from birth i have a very healthy sexual life, but from 2 years i am facing erectile dysfunction only when i try missionary position. when i visited the doctors they said you are absolutely ok. can you tell me please what is the problem, if i am alright then why i am facing this problem with this position from 2 years.Please advise.