Henry wrote:
If the wall material is crumbly and you end up with an oversize hole a
trick that I use is to vacuum out the dust from the hole, fill the hole
with a quantity of hot melt glue, and then push the plug into that. Let
the glue set and bingo - a very strong fixing.

That sounds very dodgy, because HMG is soft and will creep under
mechanical load. It might not happen if the screws are pulled up tight,
so that friction prevents the bracket from sliding down the wall, but
still...


Coming back to the OP's problem, the main possibilities seem to be:

1. You may be habitually using fixings that are too small or short for
the load. Remember that the plug needs to go some way into brick or hard
blockwork, and the screw needs to go far enough in to expand the plug
and force it to grip. The plaster is no help at all - think of it as
unwanted 'packing', that the screw and plug have to reach through before
they can do their job.

2. There may be something drastically wrong with your drilling
technique, eg you're letting the drill move around so that the holes are
always conical. Alternatively, your hammer drill may be under-powered,
which always tempts you towards problem 1.

3. Maybe you're habitually using a drill bit that is too large for the
plug. As already pointed out, the plug should be a tight enough fit to
need a light tap with the hammer to get it in. Then the screw expands
the plug and makes it grip.

4. You're using those undersized, highly tapered plugs that come with
things you've bought. Don't - they're invariably useless!

If it's either of the last two, get your own red and brown plugs, and a
set of the right-sized masonry bits. This information is moulded on the
plastic tabs to which the plugs are attached. Note that each size of
plug can need more than one size of bit, depending on the diameter of
the screw.


--
Ian White