I've got cards on the board in slot 2 and 7.  I've tried a riser on slot 1 (which you've said won't work) and slot 5 - do I need a jumper wire on a 16x slot when using a 1x riser?  I've tried both a 16x riser and 1x USB style riser (both powered) with the same results but I haven't had a jumper on the 16x slot.  I don't know which pins to short for a 16x slot if it needs one.