I've noticed the same behavior, and I believe this is due to the FS Events API (that I assume Sublime Text is using on OS X to see when files change). From FS Events' point of view, the symlink is just a normal file and does not change even when you change files in the directory pointed to by the symlink.
Ideally, it would be great if ST would let you move your packages, etc. to a different location, but in the meantime I am going to try the [risky] proposition of directory hard links.
EDIT: I tried hardlinks (using github.com/selkhateeb/hardlink) and ST still doesn't pick up changes to files in my Dropbox. This works on my Windows box using a "Directory Junction" or whatever it's called. )
EDIT 2: Ack! I just noticed this was fixed in a recent nightly build: https://forum.sublimetext.com/t/mxs-max-script-suport/1743/1
It works great now!