Minecraft dispenser problem?

I’m playing on public Minecraft servers, I was building a flooded tunnel with a piston door at the far end with a lever that triggers both a water bucket dispenser and the piston door and so on.

Now the problem I’m having is that sometimes the dispenser requires a double pulse to pickup or drop the water and sometimes it works with only a single pulse. Is it just me or are dispensers bugged?

Is there any way to fix or avoid this?