Is it a hollow stud or a external wall,if it’s the latter then you could you frame fixings the same as what are used for fitting Upvc doors and windows.
They are long screws with a long rawl plug type fitting approx 120mm ,these should be long enough to reach into any block work behind.
The self drilling fittings and spring toggles are normall designed for holding lightweight stuff ,however if your fully loaded coatrack weighs anything like mine does then I’m not surprised they have pulled out.
Another option is to cut a slot into the plasterboard (which you will have to make good after) to slide a wooden batten in behind the plasterboard which you can then screw through the front of the plasterboard to hold in place.
Once located you can decorate back over exposed screw heads & access hole before then fitting your coat rack using long screws to reach through to the newly installed and hidden batten.