You'd need to look at the logs and/or console at the time of the failure and tell us what those say before anyone can help. Unfortunately, "failing to mount" doesn't say enough ... [by TrevorH]
on 03/07/2013 – Made popular on 03/07/2013
In my previous Ubuntu installation on my laptop, I set up the CIFS mounts in /etc/fstab such that they did not automatically mount at boot, and the regular user could mount them at will. This was to keep the system from reporting errors if the laptop was not on my home network where the shares are located.