Tension on the Tour

So I've been spinning the blue merino on Wendy, and the bobbin looks about half full, although it's probably more like a third since the more yarn there is on the bobbin, the bigger the circumference of bobbin-plus-yarn. And I'm finding that I have to adjust the tension to get the yarn to wind on before it gets overspun, which I think is pretty normal as a bobbin fills. But I seem to need to adjust it every couple of minutes, and the maiden is now definitely tilting outward away from the drive wheel.

The drive band is a piece of linen string, which I wouldn't have thought would stretch much if at all, but apparently it does. Can I fix this problem simply by using cotton string instead? Or will it stretch too? At the rate I'm going I'll be lucky to spin up what's left of the first braid, never mind starting the second.