Plants could be rootbound try to transplant into a larger pot.
If you are scared to pull the plant out of the pot then just cut off the bottom of the current pot and set it into a 5-gal. bucket full of soil or something. (make sure the current pot goes into the 5-gal bucket so it cant tip over and cause major damage though)
Rootbound will cause yellowing of leaves and will kill the bottom leaves and work its way up but im sure you have time. Also let us know what the PH of the soil is and maby give it a flush if you havent already.