With a baby and the way they move about all the time, it's very hard to get the bags to stick for the amount of time they're 'supposed' to. :( From reading another board for parents of ostomy children, it's very hard to find anyone for whom the bag sticks longer than 24 hours on their babies.
Try warming the wafer up first with a hairdryer on it's lowest setting. The warmth makes the wafer stick better.
Make sure the skin is completely dry before applying the wafer.
Another suggestion from the other board is "We used the Convatec Little ones until A__ was 5-6 months old. Frustrated with the inconsistency of the pouches, we switched to Convatec's adult one piece pouches, and they have worked wonderfully."
Finally, contact as many ostomy manufacturers as you can and get as many free samples as you can to try out. You may find that switching to another brand does the trick.
Good luck to you and your little one.
I'm not a complete idiot - some parts are missing!
Ileostomy for 27 years since I was 10 years old, due to UC.