Try to reglue the decal. Otherwise you can buy replacements.
Also, try to move the target away from the playfield so it doesn't rub on the edge of the hole (if it is). There may be some wiggle room. Try loosening the screws which hold the entire mechanism to the playfield and push the entire mechanism towards the back of the playfield just a bit. Then re-tighten the screws. Too much and the target will rub the back of the hole.
Also, you need to clean the part of the target underneath the playfield. And as others said, make sure the spring is not stretched out.
Drop targets should snap down, not slowly lower.