I have 2 mock oranges. One is small and seems slow growing but it is full of flowers every year. My other one is growing fast and is already 6 ft high but has no flowers or so few that you have to search for them. What can I do to encourage more flowers to bloom on the tall one? I treat them the same.
Pruning the taller plant in early summer (after it's done blooming) will encourage more blooms next year.
This article will explain the proper way to prune your Mock Orange: