Answer: Since your other citrus are doing well, I'll assume you have good overall growing conditions and are providing generally good care. A pH somewhere between 6 and 8 is fine. The transplanting may have stressed the tree a bit, or you may be seeing some sort of nutrient problem.
In citrus, a common reason for lack of fruiting and premature dropping of fruit is a low level of phosphorus in the soil. You might want to try a slightly higher amount of phosphorus in your fertilization program. (Do not go overboard, just make sure you are using a complete fertilizer containing N,P,K and micronutrients.) Keep in mind it is a bit slow to make a difference. For example, if applied in the fall it should begin to show results the following season, so do not expect an overnight change.
With a new tree, however, I would suspect you are still seeing the effects of the plant not yet being fully mature and able to carry a full load of fruit. Orange trees generally drop most of their fruit, bringing maybe less than 10 percent to maturity so what you are seeing may be normal. Keep in mind also that a container tree will not bear as well as a tree growing in the ground.
I hope this helps.
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