What is the best fertilizer to get my flowering plants to FLOWER well?

Hi, I have tons of flowers and flowering plants that are beautiful BUT they do not flower a great deal. I do use my own compost but am wondering if there are any other natural things (as compost is) to produce flowers. Now I have heard that when picking a store product that one should always look for themiddle number of the three to be the greatest. True???? Thanks, Karen