Matt, the sun came out and I was able to take a couple photos that should help you get your AXA Defender lock tight on the seatstays once again.
I'll probably over-explain this, but the intent is to explain the "whys" and so make it easier to do. This would be so quick'n'easy to explain if we were talking over coffee or tea at the corner shop.
Okay...
Photo 1 (below) shows the lock with the cover plates I was talking about (the left one removed, the right-side one in place). These are only used if the lock is bolted on to a bike (typically a Dutch bike) that is equipped with mounting bosses on the seatstays /or/ if one is using the nutted ATB mounts (which I have called "cup hooks" 'cos that's what they look like). The covers are cosmetic and simply cover the mounting bolts or nuts and make the lot look more finished. If you need to remove the covers, a gentle pry with a small-blade slotted screwdriver in the slot will pop them free. They snap back into place. I don't think your lock has these.
Photo 2 (below) shows the setup I think you have, with what I call a strap-winder on the left, and a cover plate on the right. Yours would have two strap-winders (because the winder /or/ the cover plate fit in the same space, you can only use one or the other, but not both at once).
Photo 3 (below) shows the strap-winder mechanism with a strap in place. You can purchase straps separately, winders separately, or in pairs. AXA is good about making spares available. They have a great key-registration program and key replacement service available as well. You can buy fixed or spring-loaded keys. I bought one of the latter so it wouldn't be as likely to wear a hole in my pants pockets or scratch things up in my handlebar bag. I'm "careful" (frugal) and my stuff lasts a long time 'cos I take care of it)
Now, here's how it works. The strap-winder is a worm-drive gear mechanism (a bit like a windlass) that fits in the well on each side of the lock. The anchor for the strap goes in the slot in the lock and the end of the strap loops around the seatstay and goes back into a slot in the shaft that is tightened by the screw (like the rabbit going 'round the bush and down the hole when tying knots). The worm drive allows the screw to turn the shaft, but prevents the shaft from turning the screw.
Turn the screw and you tighten the straps.
Your lock is loose, for whatever reason. It needs to be tightened. If you have the little covered metal straps holding the lock 'round the 'stays, then you also have the strap-winders. Position the lock where you want it and place a slotted screwdriver in the screw slot and turn till the lock is tight.
That's all you need to do. Don't over do it, or there's a potential to damage the mechanism or strap anchors. Really firm but not knuckle-busting should do the job. If you overdo it, replacement parts are available reasonably.
I hope this helps; turning the screws should see the lock tight for you quickly and easily. If you run into problems, give a shout. Let me know how it goes.
All the best,
Dan.