I've done a bit more testing, and I've boosted without shift once. So, I guess the time I tried pressing shift was just the first time I got the down-boost to work.
Even when I'm at what I'm pretty sure is the right altitude, pressing F sometimes doesn't do anything, and I have no idea why. I'm sorry that I can't provide more detail than that. If I could...well, I guess if I understood what was going on well enough to explain it, I wouldn't have a problem.