UCX collision messed up!

Hello, I need help with the collision mesh for my model. The model is big and you have to be able to walk around on it. When I use UCX to make the collision so it fits all around the model it all most works but it has some bits that go messed up and really big. When I delete them it deletes parts of the collision that I want. Is there a reason why this is happening and or is there a other way to get on point collision.