Make sure theres no bits in the standoff that could be stalling the screws,
if necessary take them all out, use a pair of pliers to hold the standoffs and screw a mobo screw all the way in to seat, put the ones you've cleared to one side obv.
all screws should be touching the Mobo, its support as much as grounding, but triplecheck the spacing on standoffs, and that your using the correct screws
Moto