I tried a search on this subject without exact results. The trim tabs have been working all summer until today. The plate will go all the way down (pistons extended) but will not come back up. Plenty of fluid in he reservoir, any ideas? I'm thinking it might be the actual switch attached to the the steering column.
I have one fuse on my Bennett tab control. It's on the line from power to the switch. Listen to the pump to hear if it's running when you push the button up. If it's not try reversing the switch wires on the top and bottom at the back of the switch. That would let the tab up but not down if the switch is at fault. It could be corrosion on any connector or at the steering column. If the pump runs try manually pushing it up while someone presses the button up. If that works then cycle that several times.
I had Bennett Trim Tabs on a Boston Whaler...though I can't help with your specific problem, my experience is that their customer service is outstanding. If it's not a fuse, give them a call. I bet they'll be happy to help you figure it out.