Things You'll Need
Small length of wire
Wall anchors work either by expanding outwards against the hole in which they fit or by clamping against the back of the wallboard. In either case, a wall anchor has to open to exert its holding power. Anchors are designed to open automatically but they sometimes need help or simply fail to open altogether. The amount of difficulty you'll have with a wall anchor that doesn't work properly depends on the kind or anchors and screws you're using. In all cases, the holding power of an anchor depends on driving the right size hole for it.
Use a larger screw in a plastic expansion anchor that doesn't hold. The screw has to have a wide enough shank to expand the anchor sufficiently for the barbed edges to catch on the drywall. It is also possible that the pilot hole you drilled is too wide. The hole should be small enough so that you have to tap the anchor into place with a hammer rather than simply push it in by hand.
Open the wings of a winged plastic anchor by inserting a wire or awl after you have place the anchor in the wall. You can buy a special tool for this purpose at the store where you purchase the anchors.
Drive the bolt partially into the sleeve of a metal toggle bolt with a screwdriver after you have inserted the sleeve in the wall. Pull the bolt out to expand the wings against the back of the drywall and then hold the sleeve out with pliers while you continue drive the bolt. Release the sleeve when the head of the bolt is about 1/2 inch from the wall and then drive it in the rest of the way.
Use a longer molly bolt if you are having trouble driving the bolt into the sleeve because the sleeve won't expand. If the molly bolt is the right length, the end of the sleeve expands outward and locks against the drywall when you drive the bolt. If it is too short, however, it pushes against the edges of the hole and won't open, making it difficult to drive the bolt.
In some rare cases, an old rusty toggle bolt may not open until you spray it with lubricant.
Your molly bolt may be correctly sized for the thickness of the drywall, but there may be something behind the wall increasing the thickness.
While you shouldn't rely on any wall anchor that hasn't opened to support weight, be especially wary of plastic expansion anchors that aren't firmly seated. They may appear stable but may gradually slip out when subjected to an outward force.