stinky, with any limbsavers: You will find that you cannot ever really get them tightened down to where you can't turn the screw anymore. It just doesn't happen. This is mainly because of the rubber that is in contact with the limb itself. This is just how they work.
Now, you could have a defective set, but what probably happened is someone tried to tighten them down TOO far, and they are now broken. I have never seen anyone have them slip like yours are. As stated above, Simms will still probably replace them for you.
As far as placement on the limb, we usually put ours at a point on the limb where the limbsaver is approximately 1/2"-5/8" away from the buss cable. Just make sure you don't put them too close where they could come in contact with the cables.