Nice scope but won't stay sighted in - Optima AO 3-9x40mm Rifle Scope

I have 3 of these I bought 2 and one had a lens come loose after about 25 shots they replaced it no questions ask so thought every thing was good and I purchased another one. Now after around 500 shots 2 of them will not hold zero they shoot 4 to 6 inch groups at 20yds the other one still shoots 1/4 inch groups at 20yds but how much longer will it last? I have 4 of the 4x12x50AO Scopes they've lasted several years so far and they shoot around 1/4inch groups at 20 yds.