You have to supply phosphorous to the plants, not just nitrogen and potassium. KH2PO4 is a convenient way to do that. Removing it from your dosing is a mistake.
You very likely have extremely high light, with a 28 watt screw-in CFL bulb, vertically mounted in a "dome" type reflector, 10 inches from the substrate. Raise it to about 20 inches from the substrate, or use a 9 or 10 watt bulb about 15 inches from the substrate. Without seeing a picture of the reflector I don't know if it is a good one or not. If it isn't a good one you won't have nearly as much light.