Diagonal Fell Line
Several experienced weavers in our guild have had the same problem on different looms: despite apparently even tension across a warp, the fell line quickly becomes diagonal, and one edge of the piece weaves longer than the other. We have checked tension, squareness of all the beams and beater, even whether the cloth or warp beams or warp packing material were thicker on one end. Nothing. The yarns aren't anything weird either–generally pearl cotton. This doesn't seem to happen on every project on those same looms but is very resistant to fixing on the projects when it does occur. What could be wrong? Thanks!
If the fell is slanted toward the front beam on the same side as the beam crank, then my guess is that there is torque in the beam. That is, its turning circumference at the brake/crank side is slightly smaller than at the at the far end of the beam. The consequence you are observing, though, would also depend on the tensioning method you are using. If someone is holding the warp under tension as the beam is turned, the tension is more likely to be uneven at the two ends of the beam as described. If you tension the warp after turning the crank by pulling on sections of it (the method I like and show in Warping Your Loom), then tension differences like this shouldn't occur. There are two disadvantages to holding a warp while it's wound on the beam: this one and also greater tension on the outer edges of the warp that happens because you can't really apply the same tension to all the threads when you are holding many threads in your hand.