Answer: There are only a few reasons for non-blooming in forsythia. The first is pruning; forsythia blooms on wood produced the previous season so your practice of pruning after bloom is finished is the right approach. This gives the plant all summer to produce flowering wood for the following year. The second common cause for non-blooming is inadequate sunlight exposure. Make sure your forsythia receives all day sunshine during the summer months. Finally, over-fertilization will cause lots of green growth at the expense of blooms so don't fertilize your shrubs.
If you've repeatedly replaced a shrub, I'd suspect a problem with the soil. Since you're sure the soil drains well, you might want to have the soil tested for residual chemicals. Aside from the above, I can only encourage you to keep trying. Forsythia is such a beautiful plant that its worth the extra effort.
Best wishes with your landscape!
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