Siala - one of the arms of mine is a bit looser than the other, but that's something that happens with all glasses (my husband was an optician and that's just something that happens - he says it also is due in part to the wearer - if you take them off with one hand as opposed to 2 (a bad habit I have that drives him nuts) one side will probably loosen up a bit more). It's easy enough to fix though - you can buy one of those cheap eyeglass repair kits from a drugstore or supermarket and use the little screwdriver that comes with it.
Thanks Willowsmom (and Willowsmom's husband!). I carry a little eyeglass repair kit with me and tighten up the screws on mine a couple of times a day. Drives me nuts, though. I'll definitely be more careful about taking them off with both hands, though, instead of one hand. That's a great tip!