The more scouts see you the more they can see your flaws. I remember guys like DeVon Hardin, Josh Heytvelt, Trent Plaisted, Tyler Hansborough, Josh McRoberts being heavily touted as mid to late first rounders only for them to stay in school and have their stock plummet dramatically into mid to late 2nds. Look at Roy Hibbert- projected top 5 pick in 07- stays in school and drops all the way to 17th in 08. Look at the draft right now and Hasheem Thabeet is basically the only player that could have gone in 07 or 08 whose stock has risen. Joakim Noah could have been the first player chosen in 06 but he stayed in school and dropped to 9 in 07. Players stock doesn't generally raise if they stay in school.
In some cases yes, but there are probably 10x more players that increase their stock than decrease it. My guess is that since Brackins is relatively still an unknown at this point (where Hansborough, Hibbert, Noah, etc. where huge names), he has a much better chance at increasing his stock with another good year than he does having his stock fall.