In my experience, the original and replacement tracks made by Zodiac stretch rather quickly and then slip. I've been buying aftermarket tracks and get longer life. Possibly a wee bit less traction, but overall much happier with the aftermarket tracks for MX6 and MX8, including that they're half the price. I bought them off ebay.
My local pool store recommended I do a rebuild with the rebuild kit, and that something wears out that causes the track to be loose. That was entirely wrong. The cleaners just needed tracks that stayed tighter for longer. If it's only three weeks old, I can't imagine that it would need the rebuild kit, but I suppose that's also an option.