It could also be the light. What kind of light do you have and what rating??
PS--I do not think you need to run the CO2 after the lights are off. I would keep the start time, but (IMO) have the CO2 go off when the lights go off.
My buddy who does all planted tanks set this up. The reason being is that plants do need some time once the lights come on to begin photosynthesis. In order to get the best results from his experience, the plants warm up for a bit and then we add CO2. The plants will absorb it better and you will see increased growth. I believe that he got this right since the growth on the plants is crazy fast.