Your pistons are too close to one another; there must be at least 14 pixels between them. The problem is that the pistons are blocked by their neighbours to the right. 14 pixels is how far the frame of a piston reaches at most; by moving the next piston beyond this reach, you fix the blockage. I agree that creating long pistons like this is tricky; it's not often done so, as a result, this problem doesn't often come up.

Btw the horizontal INST line of the extension circuit is too close to the NSCNs of the retraction circuit, causing spurious retractions and other breakage. Move it up by one pixel.