Try a fertilizer which also contains essential trace minerals like:
molybdenum, manganese, magnesium, etc. Some of the Miracle Grow
fertilizers contain most of the main trace minerals.
My wife had one plant for 12 years, and it just barely lived, and didn't
look much different over the years. Then I gave it some minerals, and
BANG - it started growing like crazy, and the new leaves had a much better
look to them - like you would see in the commercial greenhouses.
Sometimes plants can use up some of the minerals that are in the potting
soil, and if they aren't replaced, they don't grow as well - or even die.
Kevin Hass WB0DPN