I'm growing strawberries and I've added phosphorus and nitrogen and the plant is growing great! And it'll bloom and you can see a green start of a berry - then it just stops growing??? Same thing with my plum tree - extremely small fruit and even if it had ripened it was not nearly the size I want. What is my soil missing??

I never give them any fertilizer.

Poor pollination is your problem.
Also try to keep the nitrogen under control: unless you are growing large leaf cabbages, always use a balanced fertilizer, or switch over to a PK (nitrogen free) fertilizer when development reach the "critical stage".