First things to check:
Does the aegir user have proper permissions?
sudo chown aegir -R PLATFORM/sites/yoursite
Is the database properly set up? Try drush sql-cli to see if the site db user has access and if there are tables (try SHOW TABLES).
If something goes wrong with importing a site you might end up with a node for a non-working site. In such a case it can be good to delete the node for this non-working site and verify the underlying platform again. Since there is no direct way to delete such a node from the user interface you have change the URL from node/NUMBER/edit into node/NUMBER/delete to do this.