I found the osfm were too big on my newbies. I couldn't get a good fit with them until about 3 -4 mo. Mine were GKs though. I preferred the sized ones for that reason. It is a lot of trial & error but you'll get there. The IB's might just need a bit of adjusting to get the right fit and/or extra washing like bskilner said. With the fit, just make sure it is firm around the legs but not too tight. If it's too tight, there's no room for the poop & it will escape. Too loose it will find holes. If it's wees that's leaking, it's probably more an absorbancy issue & may need more boosting. HTH