One thing about container gardening is that the soil needs to have some kind of fertilizer added regularly. This is because the soil dries faster, requiring more frequent watering. The more frequent watering will cause the nutrients to be washed out of the soil over time.
I have found that replacing half the soil in a container with new soil or compost or worm castings every time I replant the container works well. Even with that, I sometimes add a half- or quarter-strength mixture of a fertilizer every third or fourth time I water the plants in the container.
You can use an organic fertilizer, if you wish -- I do when i intend to eat the plants, i.e. herbs and such -- but I use other types if the plants are merely decorative, such as my African violets.
So if you haven't changed the soil, or at least part of it, each year, or alternatively supplemented with a fertilizer, it may just be that there are no longer sufficient nutrients to support the plants during the process of producing flowers. In the plant world, producing flowers is sort of like an animal producing offspring. It requires more energy than just producing leaves.
It's just a suggestion of one thing that might be affecting the blooming of your plants. Something to consider.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams