Hello. After reading you post, I think I might have some solutions for you:

The overwintering in your garage may have set it back for a long time. I have experience with cold weather because I live in Edmonton, Alberta, Canada where winter is 8 months of the year. I have 12 hibiscus trees and cold weather below 45F can set them back for 6 to 8 weeks. I can only keep mine outside from the end of May to the end of August because of freezing temps.

Try this:

Sunlight: Hibiscus do not necessarily like a full day of direct sun, especially after being in the garage all winter in the dark. Make sure it gets at least 4 hours of direct or filtered sun, but keep it out of the direct sun and heat while it comes back to blooming.

Food: I have tried several fertilizers and supplements over the years, but found that only high quality plant food in liquid form yields excellent results in every one of my specimens. I have been using Botanicare pure blend pro bloom for soil food with excellent results. You may want to try feeding it this in once a week doses.

Overall, you just have to be patient. I had struggled with mine in the past and found that after colder weather below 45F, that some, but not all hibiscus, may take a 6 to 8 weeks to bloom again even if they are growing heavy foliage.

Hope this helps!

Cheers,

Gord

I have had the same problem with two of the tropical hibiscus. I have 6 that are blooming their heads off and two that are treated the same, same light, water, fert., and have not done anything all Summer. Could it be the cold evenings, and wetter weather in Ky. this year?
nana8