Activated carbon has little impact on plant or algae growth, it doesn't effectively remove the nutrients required by them. BTW, you shouldn't need to be using activated carbon on a regular basis.
While I can't tell you why you're not getting algae, I can point out potential problems for your plant growth.
The most obvious thing is the CO2 and fert level. For a 55g tank, a DIY setup is insufficient to provide a stable concentration of CO2. Since you've lowered your light you can get by but I highly recommend saving up for a pressurized unit.
You need to start dosing the full range of ferts more than once a week, at least twice a week at 2wpg. macros (nitrates, phosphates, potassium), micros (trace minerals like iron, copper, boron, etc), and possibly calcium and magnesium.
While I can't tell you why you're not getting algae, I can point out potential problems for your plant growth.
The most obvious thing is the CO2 and fert level. For a 55g tank, a DIY setup is insufficient to provide a stable concentration of CO2. Since you've lowered your light you can get by but I highly recommend saving up for a pressurized unit.
You need to start dosing the full range of ferts more than once a week, at least twice a week at 2wpg. macros (nitrates, phosphates, potassium), micros (trace minerals like iron, copper, boron, etc), and possibly calcium and magnesium.