Why do I find it so difficult to fix things to a wall . . . . . . .

[RrogerD]

I put a coat rail on a flimsy plasterboard wall with the expanding fixings, and one day one too many heavy coats were hung, and the whole lot fell off, ripping huge holes in the plasterboard. Maybe those metal fixings would have worked though.

No, that was exactly my point above. Do it once, do it properly. The large, spreading fixings will obvs spread the load but at the end of the day plasterboard isn't very structurally sound. Use long screws and fix to wall behind I say.
Only thing I'd hang from plasterboard are pictures and light ones at that.

 

[JP_]

I suppose of the void is really big, you can cut out plasterboard and fit a nice piece of wood direct to the wall, then put hooks on that. Or a rough bit of wood that's a bit smaller than the plate for the hooks. Then you can hand 100 heavy coats on it with no problem.