My friend had the same issues when her ds started to be more mobil. I'll have to ask her what she did and get back to you. However my 3.5 month old ds has the same issues with pees sometimes. It seems to happen when he doesn't have to go too bad. He squirms and when I put his prefold back on he pees. It is frustrating but I don't want to push too hard and make this a negative experience. My only advice for now is don't give up trying and to maybe try a floor potty - one that he can go to when he wants. If you haven't used one before it might take some introducing to first. Otherwise I'll have to see what my friend did to overcome this issue.
Until then!
Until then!