25 trying for 18 months feel stuck

I was a young mum I have a 6 year old girl from previous relationship also my partner has 8 year old girl from previous relationship decided to come of all contraception and said when it happens it happens but seems to just not we have sex when I ovulate intact we have sex all the time doctor tested to see if I was ovulating properly and I am although I have a folic acid deficiency has any one else had this problem does it effect getting pregnant I don't understand we're both young and healthy just want to complete our family feel like I'm not fertile 😕