There is a bug with dispenser about picking water back in.

If the dispenser is powered via a redstone line going on the block above the dipsenser, the dispenser might not pick the water back.

On the picture I attached, I powered a line of dispensers with a line of redstone going on top of them all. If I press the button, some dispensers will get the water back, and others not. the failing dispensers are always the same. BUT, if I power the line from an other button, placed somewhere else on the redstone line, the dispensers failing will not be the same ones

So one button will always make the same dispensers fail, but to each button its dispensers failing. I couldn't figure any kind of logic linking the button position to the position of the failing dispensers.

most of the time, when I try to force the water back in a dispenser, I place a button on a block placed behind or below the dispenser, but I have to click twice for it to work.

I never failed to reproduce this issue, whaterver the orientation of the dispenser.

It seems other ways of powering the dispenser do not cause this issue