deffo a leak then! could be a bag or line to one if you are confident it's not the valve block. A single bag wont cope with the weight on its own and gives an illusion of both loosing air when one has a leak
It's self levelling so the other 3 corners will drop to match the height of the lowest corner, hence the tick, tick, tick from the valve block. It's working as it should but one rear spring has a leak. Fairy Liquid/Water mixture in a squirty bottle will show you where from.