I notice my rotary hook machines are a lot more likely to jam when starting if I don't carefully pull up any slack thread before starting.  Even when I do I still have a snag from time to time right as the needle comes up from the first stitch.   I try to always stop and start with the take up arm at the highest point.  When starting to stitch any slack in the thread will get snagged by the hook on it's second rotation on my rotary hook machine where of course I don't have the same issue on the