I don't know if anyone else has had this problem, but I have one of those adjustable power supplies that @HK_Fuey
mentioned. When I first got my Jaebo WP40 it was way too powerful and was causing sand dunes and bare patches in the tank, so I bought one of these adjustable supplies and ran it on a lower setting. Over time I started to increase the output and eventually ended up back at 24V and eventually found a sweet spot where the sand wasn't being disturbed.
Just recently, I stripped the tank down to move it and when switched back on, my power heads weren't working.
I tried my old fixed output power supply and it worked....but boy what a difference!
Loads of movement and I could hear a difference in the sound from the motors.
I don't think I'd realised it but over time the output from the adjustable supply must have been diminishing.
The difference was obvious straight away.
So if anyone has one of these it might be worth keeping an eye on it.