Check your KH (carbonate hardness) level. Often when co2 stops diffusing properly it is because the KH has fallen too low and it is being diffused out of the water too quickly. (lack of a buffering agent)
For you tank size and the amount of plants you have, you should be keeping the KH at a level of 8-10 degrees (12 max) and the pH should be about 6.7-7.0. I also see that you are running a bubble wand in the background. This flat-out defeats the purpose of injecting co2. Your plants will produce enough o2 to sustain fish and there is really no need for the bubbler.