Under-inflation is almost always the problem here, definitely ensure your kite is pumped good and firm. You can also try moving your back line attachments further forward (away from wing tip). Sometimes in a loop you will be prone to over-sheeting the kite for want of power, so try to remember to let the bar out as you come through the loop. This allows the kite to climb more quickly out of the loop and ‘catch’ you more effectively, and also puts less stress on the wing tips while at maximum power.
That said, you should not really be able to buckle the kite during a loop. If you are then you are likely underinflating as mentioned, or just ride SUPER hard and can generate a ton of resistance against your kite. I can’t as I am only 140 lbs!