This is an unusual situation, several things may be wrong, but it hard for me to say,
pH 5 is more acidic than ideal for Citrus, or most plants. You could try to adjust that. If you are fertilizing your tree, do so at the rate recommended on the fertilizer package. That should be enough.
I would tip the rootball out of the pot and see if roots are growing well outside of that yellow clay center. If they are then perhaps the plant has overcome that problem, but if not perhaps some of that clay could be gently knocked away and replaced with potting soil.