From what you have said and looking at 1 of your pictures of the plant I would say your plant is lacking in Nitrogen, The Symptoms are exactly as you described here and they include poor plant growth, and leaves that are pale green or yellow because they are unable to make sufficient chlorophyll. Leaves in this state are said to be chlorotic. Lower leaves (older leaves) show symptoms first, since the plant will move nitrogen from older tissues to more important younger ones, my advice is to switch to a decent NPK fertiliser, brands I have used with good results are House & Garden, Canna and Advanced Nutrients, good luck and hope this helps.