Kelp+XP farm broken in Minecraft Bedrock

I built a kelp farm (bedrock 1.16.210) like the one described in this video

It was working fine and it suddenly broke. For some reason, when the piston that breaks the kelp retracts, the water does not have time enough to refill the space so the kelp does not grow.

Any idea how to fix it?

Solution 1:

I fixed the farm.

1. I added a flip flop so that the pistons are activated once for each 2 clock cycles. The clock can work either in 1 click or 2 clicks, so the pistons are activated each 2 or 4 clicks.
2. I removed the slabs and placed the water one more block above (that is 2 blocks above the pistons) Enjoy your XP!