Answer: Although 20 years is nearing the outside end of the typical range, it is not unheard of for this plant (when grown from seed) to take that long to mature enough to begin to bloom, so they may simply still be biding their time.
Another possibility is that the spring frosts have damaged the buds year after year, although if other vines in similarly sited locations in your neighborhood have bloomed regularly then that would be ruled out as a cause.
Also, this plant blooms on old wood, meaning the growth made the previous year. So excessive pruning in summer, fall, winter, or in the spring before bloom time would decrease or eliminate the flowers. The best time to prune is in spring right after the plant blooms (or would have bloomed) at which point it should be cut back very hard. As the season progresses a bit of tip pruning can be done to try to control and direct the growth, but drastic pruning should be avoided. Certainly this means the plant has to be provided a very strong and sturdy support since it gets so big each season.
Finally, excess shade will reduce blooming (the plant prefers full sun) and excess nitrogen fertilizer will reduce blooming and cause excess vine growth. Some authorities suggest root pruning, meaning slicing downward with a sharp spade close to the vine on one side to sever half of the roots and thus shock the plant into feeling threatened and stress it into blooming in an effort to produce seed and reproduce itself -- but I have not personally seen this succeed.
Maybe, with a little luck, this will be year the vines finally bloom for you! Otherwise, there isn't a whole lot you can do, then and again patience is said to be a virtue.
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