Glad you used stand-offs. Now the details I was thinking of. Usually cases arrive with stand-offs pre-installed. Each is a metal shaft about ¼" long, with a threaded shaft out of one end that screws into a threaded hole in the case's back plate, and with threaded a hole in the top to accept the mounting screw through a mobo hole. But you will find that the case back plate has many more pre-drilled and threaded holes to allow you to re-position the stand-offs according to the requirements of your mobo. Many common mobo designs have 9 mounting holes - 3 lines of 3 each. Each hole has a small pattern of metal "fingers" around it like a little flower. The intent is that the mobo should be grounded to the case back plate via the mounting screws and stand-offs at these points ONLY. So, ideally, you should have one stand-off under the mobo at each mounting hole location for good support. But MORE IMPORTANTLY, you should NEVER have a stand-off under the mobo where there is NO mounting hole; such a mis-located stand-off can short out traces on the mobo bottom surface.
If you are not sure you did this, re-check the position of ALL your stand-offs to ensure there are none in a wrong location.