This is one of my favourite basket combos: Trixi Blueberry Parfait. They were bought as 10" hanging baskets and transplanted into the 12" tiered baskets. As the plants grew, the root systems grew too.
By end of July, I noticed fewer blooms and very little bud formation. I had slow release fertilizer in the baskets, but it didn't seem to be enough. I started using a liquid fertilizer weekly, but after the second week, no buds. I realized that the soil was so compacted, the nutrients were not being absorbed by the plants.
The fix: Using a small metal support spike, I stabbed the compacted soil medium in the three baskets. This aerated the soil and allowed the fertilizer to reach the root system. I noticed new flower buds within 2 weeks.
If you have a basket in late season that seems to need a boost, try aerating the soil medium to keep the flower power going.