Any plane will snap out of a tight loop or tight banked turn when it reaches the G load where the wing stalls. The cure is to open up the looping radius or to make the model lighter so the wing loading is less and it stalls at some even tighter looping radius.

Slowing down and doing your loop at a slower airspeed will often help as well. The higher airspeed generates more G load for a given radius of turn or loop.

The suggestions given by the guys above will all open up the looping radius in order to cure the issue. But there are a couple of things you can try which will aid in delaying the stall and perhaps let you turn tighter than you are now. Maybe not much tighter but they are worth trying.

One thing you could try is to add turbulator strips along the upper surface of the wing at about 10 to 15% back from the leading edge. How thick and what shape you use depends on the size of the model. I'd suggest you try a layer or two of 1/8 wide masking tape for now. You want to keep the turbulators fairly narrow. Another fellow in a thread running just now made some sawtooth turbulators from vinyl trim and this option worked for him. The stripe being about 1/2 inch wide with a 3/8 or so diagonal saw tooth pattern. If these don't work try some other shapes or size of sawteeth and try moving it even more forward. Also all else being equal a more rounded leading edge will delay the stall. If your leading edge is really sharp try rounding it to a bigger radius. A little goes a long way here so don't go from razor sharp to dubbed right off. If it's razor sharp try rounding it to a 1/16 to 3/32 diameter.

If it is easy to set up for some flaperon mix then you could try mixing in a slight droop angle. Up to around 5 degrees will add camber to the airfoil which allows it to operate at higher lift coefficients before stalling. This MAY delay your wing stall and give you the ability to reach higher angles of attack in your turns or loops without stalling. But the last thing you want to do is add reflex. That would have the opposite effect and allow it to stall sooner. Also this will only work if you have full span strip ailerons. If you have outer barn door ailerons then you're hooped. Adding some reflex to THOSE would aid in delaying the tips from stalling because the tips are operating at a lower angle of attack. But the center is then being asked to work even harder so it'll stall sooner. Althernately drooping the tips would unload the center and load the tips and they would tend to snap really big time when they do finally let go.

If this is a pylon racer then your turn radius success starts right on the building board. LIghter is righter and a lower wing loading will give the tightest possible turn without stalling out. Airfoil counts as well. You want enough camber that you gain a shift in the lift curve on the positive side but the low drag zone of the lift to drag curve where the line is the most vertical should reach the Cl=0 line so that you have low drag in the straight portions. The use of flaperons CAN shift this curve but often the rules do not allow "lift enhancing devices". So check that.

Other than these things the only other way is to just give in to the laws of physics and open up the turn or loop radius by using less elevator throw.