Use the right browser
Use Google Chrome or Safari 5. Firefox is fine too, but make sure it's version 3.5x or higher. Chrome and Safari are much faster than Firefox so they are worth taking the time to install.
Make sure the your computer's time is right
WebPDI uses your computers time to discern what tab week to generate. Your computer's time must be set correctly.
Acquire a quality internet connection
I highly suggest you invest in a quality internet connection for your office. More and more systems and operations used in a Vector office are moving to the web. Having a fast reliable internet connection has the utmost importance.