I wonder if the problem is with story structure rather than writing. You say your short stories get a good response but then novels fall flat. No amount of gorgeous word wrangling will save a story that lacks drama. Maybe you need a developmental editor to have a look. It'd be expensive but it could be worth it if they spot the problem.

Alternatively, perhaps your stories are too niche and an agent can't see finding a big enough market. In which case you might be a good candidate for self publishing if you can see yourself doing the necessary marketing work.