"What reasons can you think of for her lack of success?"
If Nelson George is to be believed, writing in his "Where Did Our Love Go" book, it was because she couldn't sing well .. not that I agree with him at all.
It would be the usual suspects ... lack of promotion, too much competition from other artists and labels, and US radio stations not knowing whether to categorise her recordings as "Black" or "White" (as also happened with Ike & Tina Turner's "River Deep Mountain High").
Having said that, I find it very surprising that her "I Want To Go Back There Again" wasn't a major US Hit.
Roger