I've found bee pollination can be iffy if the tree doesn't have a high
enough concentration of flowers for the bees to notice it. I was told once that it can take a 4'x4' patch of the same type of plant to be sure the bees will find it.
For that reason, I admit I'm the pollinator for my peach tree - it's not large, it tends to bloom in two goes and depending on the weather, I never know which go will produce fruit for the year. I pull a blossom from a cluster and holding by it's stem just push it against a bunch of other blossoms sequentially and hope for the best!