Using PayPal Sandbox for Fundraising

When using the PayPal sandbox and clicking "click here to complete your donation" I am taken to the thank you page and it says "Oh No, we can't find your Pledge. Sometimes it take a few moments for your Pledge to be logged. You can try refreshing this page "

Nothing gets registered in the admin area in terms of pledges, supporters and dollars donated.

Is there a way around this? I'm supposed to demo this for a client tomorrow.

Any help would be hugely appreciated.
Steve Stadwiser