We had this issue lately with my oldest son, who on top of waking up with wet sheets/pjs from time to time--would also on occasion have a totally dry pull up/diaper! *maddening!!*
As others have suggested already, I'd reccomend trying the Huggies overnights. I wasn't aware they even existed until I saw them mentioned on here, and since we've been buying them I haven't had a single issue thus far.
Another thing I've found to help with primarily belly sleepers (although it seems to help a little bit with my other son, who sleeps tummy up), is to put the diaper on a bit higher up on his waist before he goes to bed. Not so much to where it fits incorrectly or is on him all crazy--but just enough to add a bit of a safety net 'up front'.
P.S- before I could find the Huggies in stock in the size we needed, I found the generic brand of Walmart overnight diapers (parents choice I think..?) worked quite well. I've heard from a friend that the regular diapers aren't particularly good--but the ones specially marked as overnights worked quite well for DS in the meantime.