A treadle cannot be pressed.

Possible causes:

1. The locking pin is still in the parallel cords

2. In the case of countermarch: An error in tying up the treadle; the treadle is tied to both the lower and upper lam of the same shaft.

3. Check if there are cords or heddles that are obstructing the movement of the shafts.
In the case of countermarch: one of the cords runs in the slot of another shaft.
If you have just moved heddles or released shafts, check that all cords are running correctly.

4. Sometimes with the David, a treadle tie-up cord with a loop gets caught on one of the screws. In that case, the shaft will also be higher (or lower).