i used 100mm by 6mm hammer in fittings to reach the solid wall behind the plasterboard.
Alternatively, what you can use which is what I used for the really heavy curtains down stairs....i used....can't remember their names...I think compression fittings.....basically a hollow metal tube it looks like.
I got ones that were 8mm and 150mm long. so i drilled for them, they easily reached the solid wall in behind. Before I hammed them in, I gave it a few belts of a hammer in say 2 places to tighten the tube slightly. I then basically used this metal tube as a receiver for the 6mm 100mm fitting. The rawlplug element of that fits in very snugly to the tube, then when you screw in the tube to it, it makes a rock solid fitting and can take a hell of a lot of weight.
For light rails though you should have been able to use those spikey metal anchors. You hammer the tip into the plaster board, screw in the fitting and then screw a screw into it so its the plasterboard taking the weight...i'm guessing that they are the ones then went wrong???