Leicester Dryad - no shed

I have bought a used Dryad counterbalance loom from a lady in Devon who used it for years on a variety of projects (which I have seen), so I know it works, but for some reason I'm getting only about an inch of shed when I treadle it.  It is tied as plain weave for both left and right feet (2 pairs of 1-3/2-4). 

It looks like each warp thread is pressing against the top/bottom of the adjacent harness as the shed opens so I can barely tread it at all before it stops.  The lowered threads are a bit tighter than the upper but the warp is hitting in both directions and doesn't move far enough to touch the bottom or top of the reed dents.  I have gone underneath to make sure there is no lateral interference in the strings to the lams (in fact I have retied multiple times hoping to see some improvement).  I wouldn't swear they are absolutely vertical but they look pretty good.  I have some longer heddles which I was going to use eventually, as the current ones show some wear and seem quite short to me, but I don't want to assume that is the problem when I know the old ones were good enough for the seller to weave with a 2" rug shuttle.  Apart from what's happening when I open the shed everything else appears to be in order.  The strings are of uniform lengths, the heddle eyes are of a height with the beams, the horses are not yawing enough to foul each other, and the warp is sitting horizontaly and passing through the reed as close to center-height as I can get without lowering the beater beyond its tolerance.  I am hoping one of you counterbalance users will point out something I didn't think of before I resort to unthreading and switching out the heddles.