A common issue for holster makers when making lined holsters, it caused me a bit of consternation the first time it happened to me! It will happen with anything that is lined when you try to make a sharpish fold. The only way to avoid it is to pre-curve the pieces, as Dwight said. It can be a bit fiddly trying to line up two curved pieces that have contact cement on them but I do as he also said and use stiffish plastic to keep the pieces apart, working it out a bit at a time.