The most common problem with the BSA plastic stock is that the stock stud is too long, as Riku said above, the only proper cure is to have it skimmed (faced) down in length on a lathe.
The length of the stock screw isn't usually a problem unless somebody has replaced the original one with a non-standard length screw, in this case a washer under the screw head will cure the problem. As said above by Woddenleg a washer hasn't cured it so either it is an original screw or a replacement of the correct length.
The stock stud is common to many BSA guns yet it seems to be a problem with the plastic stocked ones, another possible cure is to deepen the counterbore in the stock (from the stud side not the screw side) but this isn't easy unless you have a pillar drill and the correct size counterbore cutter or a suitable milling cutter.