Swapping which grey tank drain valve was open should not have 'cured' your draining issue with the shower grey tank.
What may have happened........
Shower grey tank vent to roof sticks to far down into grey tank and when water level was high enough it blocked the vent.
Then when you tried to drain thru small garden hose it became 'vapor locked' (no air coming back in vent because it was under water)
Then when you swapped drain valves some water transferred to galley tank grey tank for kitchen sink) uncovering shower grey tank vent line.
Fill shower grey tank full and try draining again.......if it doesn't drain then vent line is getting blocked by water level in tank.
