Any suggestions to get tomato plants flowering?
Annual plants have 2 main stages of growth: Vegetation, and Reproduction.
The 3 main nutrients
N-P-K (Nitrogen, Phosphorus, and Potassium) each promote different phases of a plant's life.
N will promote the early growth of vegetation, but too much will inhibit the reproduction (flowering & fruiting).
P will promote the
roots, and the reproduction system of your plants.
K is an overall nutrient that helps the plants overcome various stresses (heat, cold, drought, etc.).
Most "Bloom" type fertilizers are higher in P and K, with little to no N in the formula.
(Examples are 5-20-10, 0-10-10, etc.)
I would say that if your plants are big and healthy, you
should NOT give them any more N, but concentrate on feeding them P & K.