CO2 only off-gasses if the co2 pressure in the water excedes that in the atmosphere. in a planted tank you need to balance co2 with light. in my low tech setups i aim for maximum surface agitation, to ensure consistent co2 levels, and adjust light accordingly. you may need to drop to t5no or raise your lights as suggested earlier. if not, injecting co2 is the other option, but not ideal if you already have good air-water gas exchange.