A tip I found on here which is amazingly helpful if you have crumbly walls and loose plugs:
Drill and clean the hole
Put in the plug and discover that it is loose.
Take a tube of "NoMoreNails" or similar with a long nozzle on it (I use the cheap own-brand water-based ones for this job).
Push the nozzle deep into the hole and squeeze as you withdraw it, so the hole is filled from the bottom and does not have a bubble of air at the end
You now have a hole of suitable depth, filled with NoMoreNails.
Poke your plastic wallplug into the hole, so that the outer end is slightly below the surface of the plaster, and wipe off the excess with a damp sponge (with practice, you will learn to stop squeezing before you fully withdraw the nozzle, so that you waste less) the plug is now completely encased in NoMoreNails and there is no air gap.
(You may find it easier to push the plug in if you insert a screw into the plug and give it a turn or two, then you can use the screw as a handle and it prevents you losing the hole in the plug under excess NoMoreNails
If the plug was really loose, leave it overnight to harden. If it was a good enough fit not to spin in the hole, you can now fix your bracket or whatever and drive in the screw without leaving it to set. If the plug starts to turn, stop screwing and leave it overnight to set.
The tip was posted by gazza2 on this site not long ago. I have seen professional injection kits using resins that are more expensive
I have been DIYing since long before you were born, and have only recently heard this successful this way of dealing with loose wallplugs. I know no other way that works as well.