Causes of yellowing leaves on Ficus
I've had a Ficus Lyrata for about a month now. It's been doing well, particularly recently when I've added a CFL (5500k, 1300 lumens) above it because the light levels have been so low. It even just put out a gorgeous, huge, and very healthy looking new leaf. Just today I noticed that some of the leaves about half way up the tree are looking dull, not shiny like they were when I first got it. The lowest leaves on the tree are looking even more dull and a little yellow. The leaves at the top look fine.
The tree came to me planted in a bark based mix that I think has fertilizer pellets in it. I've been flushing with every watering and fertilizing weakly every weekly. Since its winter I've been fertilizing at about 1/4 the amount the instructions say. I use Dyna Grow Pro Foliage. I only water when the mix has "dried" (not totally dehydrated) to about three inches down in the pot.
I've read that yellowing leaves on a ficus can be a sign of under fertilizing. Of course I know it can also be a sign of under or over watering too. I think I've been doing ok with watering.
I couldn't find any information about dull looking leaves except in the presence of an insect infestation, which I couldn't find any signs of on my plant. I do think I might have seen a fungus gnat hovering around the mix last time I watered but I think that probably came from my two orchids that had been infested. Just in case I looked at photos of aphids and white flies and have not seen anything that resembles them.
I'm guessing that the dull leaves are from not enough light getting to the leaves lower down on the tree. What are your thoughts? What are possible causes of dull leaves on a ficus?