It is very persnickety. It prefers to swallow that which it is attached to before producing blooms although at times, it can issue a few blooms before getting there. Make sure you are not adding lawn fertilizer (high in nitrogen) that may keep it from blooming though.

It's more a matter of age rather than size. Depending on the size/age of the plant at planting, 5-7 years in the ground before blooming is typical, although some report the vine not blooming for as long as 10 years.