Seen it plenty of times.
There are many, many reasons for this...I'm just going to name a few...if they don't help then let me know.
1) Check the cord. Sometimes the cord connecting the computers can be bad and not connect sometimes.
2) Check your settings, in the Chooser, in File Exchange, and in File Sharing.
3) Try File Sharing with no applications open, sometimes it's just a memory problem.
Good luck!