I recently added a sectional beam to my baby Mac, and I'm loving it, but I keep having the same issue: the selvedge threads on both sides slip off to the side as I weave, loosening the tension, which leaves the threads too short as soon as I advance the warp. When I wind the beam, it's nice and tidy, just like in the pics in my weaving books (I don't have an instructor available), but as I weave, it degenerates into a hot mess. Anybody else have (and hopefully solve) this issue? I'm working with 16/2 and 20/2 linens, mostly, which have very little elasticity, so uneven tension is pretty unforgiving.