What Causes Non Conception Despite Regular Intercourse?

My wife is between 34-35yrs and we have 4 children. after the fouth child she started using injectorplan for birth control and then miscalculated the dates for the next shot.Thereafter we had sex of cause regularly and one day she decided to get another shot without going for pregnancy test. 1-2months later she starting feeling signs of her normal pregnancy then this shopped after one month. this lasted for another one month then she started to bleed and she thought she had resummed her period but only to realize that the bleeding at first was thick and dark then clear blood after. She was taken to hospital then bleeding controlled evac done put her on antibiotics. Since then she has resummed her normal periods twice but now she has not conceived despite regular unprotected sex.What could be the cause?