For me at least, this is a toughie. I only have the single, 25+ year old "mother" tree, and she's never had any real problems. Thus, I don't have any first-hand experience in dealing with what you see. I'm simply going to list some things to look for and consider:
1. Look closely for insects and mites. If you see any cottony areas where the leaf-stem meets the trunk on the underside of the leaves, that's mealy bugs. If you see fine webbing, you have mites. You might have to get out a magnifying glass to see these small critters. Mites in particular will suck the life out of young, tender leaves. The older, more mature leaves are too tough for them to harm. Whether you can identify pests or not, I would make up a solution of alcohol/water, 50/50, wet a soft cloth with the solution, and gently rub all the leaves and the leaf-stems. Do it on all the leaves and stems, not just the ones that you see damage on. Repeat this in 4-7 days.
2. When you re-potted, tell me what the roots looked like. Were they firm and not mushy?
3. Are you using tap water? I have great tap water here but many localities have very poor water quality, as least when it comes to dissolved solids. Unless you know that your tap water is excellent, switch to rainwater or distilled water.
4. This could be a water quantity problem, rather than a water quality problem. Fiddles that have a good, healthy root system, and a good leaf canopy, will use huge amounts of water. Part of that equation is because their leaves are so large that they will transpire far more volume of water than your average houseplant. Rather than simply watering on a set-schedule, look at and stick your finger an inch deep in that potting soil. If it feels completely dry, water. If it feels moist wait. If in doubt, wait a day or two. Also, make sure your potting soil drains extremely well and that there are nice, large drainage holes in the bottom of the pot. I make all my own potting soils to fit what I am growing. Thus, I don't use commercial potting media. I use a lot of coarse perlite and cypress wood chips (clean mulch) to aid in drainage. Also, water until a good bit of water comes out those drainage holes. Depending on the pot size, this might take a lot more water than you would think. Empty that cache-saucer after drainage.
5. This is a tropical tree and as such does best with moderate or high humidity. I don't know where you are located (you need to go into your "Profile"and put your city/state in there, so that your location shows when you post), but if you have central AC, the humidity in your home will be very low. Can you take your tree outside and spray it down well every day? Do you have a bright, shady spot where you could actually leave it outside for a time each day? If the tree could get a few hours of early morning sun or VERY late afternoon sun, that's even better. My plants (the mother tree and her divisions) stay outside from April till November.
They will only stay inside when the weather begins to get below 60 F. If you can't get her outside to spray, how about a shower, leaving her in the shower for a while to allow her drain and dry a bit? Speaking of AC, be sure your plant is not sitting where an AC register is blowing directly on it.
6. I would not fertilize until the leaves begin to return to normal. In my opinion, fertilizer seldom aids a plant that is suffering, when the problem(s) is generally not a fertilizer deficiency.
7. This could be a virus, but I have never had a virus associated with any of my plants (as far as I know). Fiddles are incredibly tough plants and I just don't think viruses are much of a problem. But that's just a guess on my part.
I am sorry I can't simply tell you want the problem is. I just don't have the experience. Good luck and keep us posted on what you find, what you do, and how the plant is doing. It would be nice to see more photos, showing the full plant and the pot.
