Hi all! I can't seem to get this little thornless key lime to green up, no matter how much fertilizer I give it.

Background: I bought two thornless key limes over winter on super sale, this one in the yellow pot has always been a bit of a Charlie Brown xmas tree...a spindly thing, with anemic pale leaves (effecting the whole tree- not just the new growth). I repotted it on purchase, it had a very small root system, that only grew on one side- hence its smaller pot. the other tree had a healthy root system- hence the bigger pot. Both plants otherwise receive the same care.

I think I fertilize heavily, but the plant wont green up at all. It is planted in 511 with Osmocote Plus, I topped off all of the pots with Plus again at the start of July. I use 2T FP + 1 T vinegar/ watering can at nearly every watering. At the last two waterings I switched to regular old miracle grow hoping maybe I was missing some trace element in my fertilizer...what gives?

Any help in identifying and fixing the deficiency would be greatly appreciated.