In the most gripping novels strong characters drive strong plot, and between plot and style is the rich but for many, murky area called structure.
If you're worried about your plot, you should probably worry about your characters, their through-lines and scene structure too, because that's characters and through-lines make or break plots, and plots only work when the scene structure does.
And if you're worried about those then you'll have to come back later and revisit your expression -- because good expression is about bringing out the strengths in your characters, their through-lines and scene structure. If your prose isn't bringing that stuff out then it's not strong prose -- it may just be a pleasant voice.
I can suggest three books to help you look at these:
- Scene and Structure by Jack M. Bickham -- covers basic building-blocks of stories.
- How to Write a Damn Good Novel by James N. Frey -- useful for showing how good characters drive plot-creation; builds heavily on ideas from Bickham and on notions of through-lines.
- Stein on Writing by Sol Stein -- covers the critical connection between structure and prose.
There should be enough material across all three to cover the gaps.
I probably sound like a bit of a dufus. My ending has come about organically but one agent felt it wound up too damn quickly so I guess I just need a little guidance. I think writing style can't be 'taught' - you have it or you don't - but I think plot and structure can be helped along with some decent books. So the novel's written, been revised and edited and agents have fulls but am just anticpiating a 'no' and what I can do to deal with that. My view is if agents love the writing style and synopsis enough to request a full but then turn it down, there's something wrong with the way I've worked the plot. 
