Luffa sponge gourd - no flowers

This is the second time I've tried growing luffa sponge gourds. The first time was nearly 20 years ago, and all I can remember is that it frosted before the tiny gourds got big enough. This time, I planted them very early. However, even though the plants have vined up all over my trellis, there has not been even a single flower on them.
Anyone have any ideas for why this might be? The plants are healthy and growing.
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I had a similar problem with melons. There was too much nitrogen in the soil. I added 0-10-10 and had blooms in 10 days. :~)
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