I, too, prefer fingertip-directed trackball mice. I had no success in finding a good model. (I very much dislike thumb-directed models.) Fortunately, I found my 1999 corded optical Logitech trackball that still works well.
I do have to clean it once in a while.
I'll ask a dumb question: Have you taken yours apart to see if anything is smudged or loose?
My local PC dealer had wireless optical vertical mice to try out. They are an interesting alternative -- like holding a old-fashioned wood planer. I don't remember the make or model.