I wouldnt be surprised if thats the problem, when more ring terminals start getting added its hard to tighten those flat terminal ones up, they are very soft, you'll also notice when theyve been undone a few times the connectors on the starter leads start to dish. And this would also explain why when you put your jump leads on it starts, as its touching both the battery terminal and the starter connectors.

For this reason, amongst a few others thats why i run round terminal:



Check out the size of my battery aswell lol so big you can only put the front clamp on, it fills the tray