I have had this plant for 5 years and not one bloom. I have pruned in spring and fall, fertilized, moved it to another state and NOTHING. It does grow very well.
Purple Passion Lilac does not get blooms
Stop pruning it and see that it is receiving enough light.
If you have pruned it in the fall you have simply cut your flower buds off.
Even pruning the wrong time in the spring cuts off buds.
Water it, feed it a tad in the spring, and leave it alone!
I have not pruned it for the last 3 years and fertilized it every spring and fall. It gets full sun over 8 hours per day. I have never seen a bud.
Maybe too much fertilizer? Too much nitrogen can stimulate green growth at the expense of flowers. Alternatively, any chance it's getting browsed by deer who are chewing the flower buds off?