Symlink to raspberry pi webroot breaks

I have my raspberry pi setup as an internal web server. I have an external hard drive connected to the pi and mounted correctly.

I then symlink two folders from the hard drive to the web root on the pi, one is /Movies, and the other is /Series.

Initially this all works fine, as in if I hit http://ift.tt/1FcYQt3 I get a list of all the files, I then click one and it starts playing no problem. However, after leaving it for a while and coming back, I can still get to the file directory in /Movies or /Series, but when I click a file, the web browser cannot open it.

I have no idea what's going on, it seems weird that I can still access the file directory, but playing a file doesn't work?

Any thoughts or help appreciated.