If it is like the Older ego bolts, then you will have a ball bearing, then spring then grub screw ( I think), in that order, to be loaded into the back of the bolt and done up.
It might just b the grub screw, so do it up a bit snug, but not too tight.
If it is like the newer bolts, then you have pulled the pin right out.
Put the spring, then the piston into the bolt...push it in as far as you can with something THIN.
Pop the bolt pin in the top with the machined section of the shaft facing towards the front of the bolt, then turn it 90 degrees to either side, push the pin down, then turn back 90 degrees in the opposite direction, to which you turned it.
The Piston bit should then lock up in the pin, stopping it from popping out.