Could be that your stock has compressed or that the trigger gaurd is worn . Check the locking lugs on the trigger gurd , the should be round but are often distorted . Sometimes you can peen them with a hammer lightly to get them to tighten up , but this is often only a temporary fix . You can cut a shim from a beer can where the trigger group goes in the stock (On the sides) . This will often tighten things up .
Hope this helps !