ok I see. The white graduations (bottom left) indicates sap units, you must have energy and water bars above one graduation to produce a new unit of sap.
More precisely, you need several units of water (say 3) and energy (say 5) to produce one unit of sap. So if you collect 12 water and 9 energy you still produce only one sap and blue bar is 9 units above white line and yellow bar 4 units above.