constipated 6week old

So, our 6 week old baby has had a problem with being constipated since we brought her home from hospital. while we were in hosp i was giving her sma in the pre made bottles they give you. We started her on sma gold and straight away she begain to get constipated, l mentioned this to the midwife and she said give her a lityle water or 1/2 teaspoon of apple juice and that will help. After having to do This every 2days (we could see how much pain she was in and she was just crying all the timw) we swaped her on to cow and gate, for 2weeks with the same problems as before, so we changed her again, for the last 2 weeks she has been on aptimil comfort! Still no improvement. So now I'm wondering weather to put her on sma pre made milk abd see if ttht helps or just carry on with the aptimil because shes had wnough changes with milk?! We are due to see the health visitor in fee days but just wondering if anyone else has had this problem or any suggestions what I can do to help her. Any help would be much appreciated, x