what do you do if a geranium plant has plenty of leaves but no flowers?
It either has too much nitrogen, or not enough phosphorus and potassium. You can add a high phosphorus and potassium mix, or just add potassium phosphate according to the directions.
Alternatively, you can try raising the pH with dolomitic lime. This can help the uptake of, both potassium and phosphorus, while decreasing nitrogen uptake.