It certainly has been an odd year for blossom. Here we are in the
first week of May and all my blossom has gone over. I was down at
Brogdale farm last week and their blossom seems to be behind! I would
suggest that normally you should have no problem with group A & B
overlapping. Mine more or less came at the same time making a
nonsense of the flowering groups. Having a crab apple is usually a
certain way to ensure pollination as their blossom is longer lasting

There are some apples that are partially self fertile, such as Fiesta
and Peasgood's Nonsuch
There are Tripliods and Tetraploids that require more than one
pollinator
Then there are those that are self sterile

Pollination is depentand having suitable types overlapping flowering
periods but also you are reliant on insects - so if the temperatures
are out - too cold - no flying insects poor pollination occures.

Another reason for poor set it down to feed. People plant fruit trees
don't feed them. A general purpose feed in early spring doen't go
amiss; do not apply too much nitrogen though as you will only
encourage weak sappy growth.

Regards

Clifford
Bawtry, Doncaster, South Yorkshire