Perhaps you are cutting it back too often. I find I get limited flowering once I cut it back, so I don't do it every year. I try to leave some bits so that I get some flowers.

Hello, Philadelphus flowers on old wood, are you cutting it back each year, if so this could be the reason, if you want to create a nice shape then pinch out some of the tips, come down to a few buds if desired, this will encourage a good bushy shape, by doing so flowering should occur in the coming years, over time you can keep this shape and take out some of the really old exhausted stems to encourage new stems for future flowering, also this may not apply to you but lets say if you have a very large overgown philadelphus then renovation pruning can be carried out, this entails cutting hard back, it will send new shoots up and by following the above procedure a lovely well balanced shrub will be acheived, i will post pictures of this excercise for you later in the year when i have more time.