More info would help. In the meantime here's some possibilities:
Too much fertilization, especially nitrogen. Over fertilizing, with a high N fertilizer, encourages the plant to just keep growing bigger and leafier. In this case it would be growing fast with lots of leaves, maybe light green.
Another possibility might be if they aren't getting enough hours of sunlight a day. Tomatoes are full sun plants. Plant cycles are often regulated to the season by being keyed to hours of daylight a day. If it isn't getting enough sun hours, the plant won't think it's time to bloom yet.
If neither of these seems right, then tell us some more about the plant and the situation it's in.