This is a common problem when sending sessions, because Pro Tools has the ability to references files outside of the session folder. For example, if you imported audio from another session, Pro Tools might be referencing the original files, instead of copying them into your session folder.
Please note that even if the session loads correctly on your computer, it might be missing files when you send it to me. Obviously, if you send a session to be mixed but it is missing some or all of the audio files, it will cause the mix session to grind to a halt.
The following is a fool-proof method for making sure all files are included before you send me your session for mixing.
In Pro Tools, under the File menu, select “Save Copy In…”
In the next window, in the bottom section labeled “Items To Copy”, check the box next to “Audio Files.” Click OK.
In the next window, select a location for Pro Tools to save the new session folder. It doesn’t matter where you put it, just make sure it’s somewhere that will be easy to find (the Desktop is fine, if you want).
Click Save, and Pro Tools will copy all the session files and create a copy of the session at the new location.
Close the session and/or quit Pro Tools.
Send this new folder, which should contain a new Pro Tools session and Audio Files folder, using whatever file transfer method you prefer.
Essentially what this does is copy all the audio files, no matter where they are, into a brand new session folder.