In manual mode, ‘all selected valves’ will surely all turn on, because in manual mode you click on / select each valve to turn it on. So I assume ‘all selected valves’ mean you’ve manually selected all valves, and that should turn on all valves.
In any case, what kind of transformer do you have? Normally if the transformer is overloaded (say because it can’t drive more than 1 valve), the voltage will drop instantly, causing the controller to restart, and hence release the valves. The only issue I can think of is if you have a persistent shorting somewhere, and hence the transformer will be damaged. I think it’s important to perform further investigation before arriving at a conclusion.