What's stopping it? Are the screws running against the back of the electrical box? If so you could use shorter screws or cut them off. In a pinch you could use washers on the outside that are the same diameter as the screw head, so they'd be minimally visible.

If you have typical framing, you could use a retrofit ceiling fan box to successfully adjust the position of the box. Requires removing the current box. Also check where current studs are with a GOOD stud finder. Sometimes there is atypical framing when roof trusses are involved where this idea wont work. Need box between two studs 24" max apart.