Are your trees self fertile? What varieties are your cherry and plum? Do you over feed ....perhaps too much nitrogen? Are your trees in cold exposed site? Are you pruning your trees? A pear tree will never pollinate cherry or plum. With trees growing in the garden I have bent branches down, gently, and this can encourage fruiting but with patio trees this is not an option. Perhaps moving to a warmer location and feeding only with a potash fertiliser this year will improve things.