CO2 problems can be either or both the amount of CO2, as shown by a drop checker, or the water circulation which is critical for delivering the CO2 to all of the plants. If you are using the drop checker with 4 dKH water, and it stays green, then I would assume you need to improve the water circulation in the tank. Adding a powerhead won't hurt anything, but should help move that CO2 enriched water around better. I typically see problems develop as the plant mass in the tank increases. Since I just love the effect of a tank full of healthy looking plants, different shapes and colors, I am always reluctant to prune until I absolutely have to. If that describes you too, pruning could also benefit your plants. Dense stands of plants most certainly do restrict the water circulation.