Young citrus trees ofen do not produce much fruit for a few years after planting, even if they flower well. Older trees may not set fruit at times for several reasons:

Unfavorable conditions at the time of bloom such as very wet or very dry soil conditions may also cause flowers to drop without setting fruit.

A fungal disease called post-bloom fruit drop can attack the flowers and cause the tiny fruit to drop.

Tangelos often bear few fruits unless the flowers are pollinated by another compatible variety.

Also, keep in mind that even mature, healthy citrus trees normally set from only a very small percentage of the blooms that are produced.