I can't add or load new markets to Guardian

The most likely reason for this is if you already have a load of markets which are now closed still saved in your market list window.

New ones will either be getting added to the bottom (out of view) or if you have streaming enabled you may have reached the Betfair limit of 1000 markets

In the image below you can see the market list is full of closed markets, if you look at the markets column header it tells you there is currently 941 markets in Guardian

Removing old or closed markets

To remove unwanted markets quickly and easily on the top ribbon just click either;

Remove Suspended markets 

Or

Select all Markets > Remove selected markets

Once your market list is empty you can go ahead and add your new markets.