One reason is that they may not be self fertile and you need pollinators nearbye. Preferably in your garden, that cross pollinate with your trees. For this you have to know the name of the variety. Look up the name on Google and put + pollinator into search, and it will come up with sites that cover most common fruit trees. If a tree fails to make blossom there is not a lot you can do about it.....apart from get rid of it.