I managed to find an annoying situation with poetry in which if you specify a range of versions for a dependency, and one of your other dependencies specifies a slightly different range of versions for that dependency, it will refuse to proceed. (I was expecting it to have chosen something in the intersection of the ranges, but it did not)
I suspect in GP's case there was something blocking the intersected range. That should work perfectly.
The poetry author doesn't want to support package version range overrides like yarn does [1]. His call I suppose, but I think it's the one major flaw in the otherwise giant step that poetry takes forward.