I am experiencing the same thing. I planted Grandpa Ott and Crimson Rambler. Have you had a lot of rain this summer? We have and I think that may be the reason. Usually if you water MGs too much they produce lots of leaves but few flowers. I used a Bloom Buster fertilizer last week and it seems to be working.

Not a lot of rain , but a very hot summer.I have to water or they start to dry up.I read that they like hot and humid weather which it has been. Others say that they like cooler weather.It's confusing.

I am having the same problem. I have 3 long boxes of Heavenly blue m.g's and they are constantly in heavy bloom. But, I have an ivy leaved chocolate mg that is all leaves and not a hint of a blossom any time soon. I've waited all summer, here it is pushing August, and nothing! It's been very dry and hot here, but these plants are on my deck, climbing railings and trellises, get plenty of water-not over watered, and no food. Next spring I think I'll skip mg's all together. Thunbergias are wonderful-especially blushing susie!! But why no flowers on my chocolate mg??

The weather can modulate the plant response to the lighting conditions and this can be species or cultivar specific...also the climatic factors that each strain of a species (or cultivar) has been acclimated to can certainly affect flowering...

The particular acclimation of a species strain or cultivar modulates the genetic programming to flower and / or produce seeds...

Climate (the combination of all relevant factors) and location are definitely both factors in how a plant responds...latitude and altitude cause the sunlight to travel through different amounts of the atmosphere and at different angles resulting in a different ratio of wavelengths reaching the plants...

The formula that worked for you last year may not work this year and your formula that works for you in your location this year may have an opposite affect for someone in a different location...

Sometimes there are no easy answers...