It's okay to work on flat ground when you're working on a particular pitch or delivery, but you have to get up on a mound and do the same thing. And you need to work on different heights of pitcher's mounds, because this can vary from one playing field to another. ALSO---if you're a sinkerball pitcher, you have to be sure the mound is high enough. I recall a time when Chien-Ming Wang was pitching for the Yankees against the Royals in Kansas City, and Wang was having some trouble in the early innings. Jorge Posada went out to the mound and told Wang to shorten his stride; the reason for this was that Wang is a sinkerballer, and the pitcher's mound in Kauffman Stadium is lower and flatter than in any other ballpark in the majors---and for a sinkerballer than can spell disaster. So Wang shortened his stride, and he pitched seven strong innings and got the win.