I had an issue with a new 8.1 machine (all others 7) whereby the logon script wasn’t working. Either it wasn’t running it all, or was, but with a long delay. The following links helped me solve the issue.
First link explains the Group Policy item that needs to be changed. Be aware that this doesn’t exist on SBS 2011 unless you install the ADMX files (second link) and install them (third link). As the script was a vbs that was trying to run on logon, I needed to change the registry to allow this (fourth link).