When two or more bees try to pollinate at the same time a flower that is in its distance limit of 21 blocks, they push each other causing that the first to reach the flower do not get pollen and that they return to their honeycomb without pollen.
On the other hand, if the flower is closer, several bees can pollinate it at the same time.
This is recreated by placing at least one or two honeycombs with 2 or 3 bees each one inside a greenhouse and 21 blocks in front of them place a flower, if the bees leave the honeycomb at the same time it will be easier to recreate it.