I've been using YLI invisible thread for years now. I always use cotton in the bobbin. I used to use a size 70/10 but I tried the 60/8s and I like them a whole lot better. The 60s don't leave holes like the 70s do. You do have to be very careful when doing turns not to break the needle since they're fairly fine.
Make sure when you're using an invisible thread you have the spool upright and the thread is coming off the side of the spool and not the end.