Hi Steve,
I’ve run into that problem myself. These screws are usually either straight shoulder screws or tapered under the head which fits into a matching tapered hole in the lever. Also some levers fit over a raised shoulder or post on the plate. In any case the shoulder should be longer than the thickness of the lever or the lever will loosen the screw. Also could be a burr on the plate, underside of the lever or nicked screw head or bent lever… anything that might take up the small space intended to keep the lever free.
Hope this helps Steve,
Bob