I would love to help you out as best I can!
There are a couple of general fixes that we look into when publishers are having trouble with ads filling on the page. The first step is ensuring that the site is approved within our exchange. If it’s not, no impressions will serve. Your site has been approved so this is not the issue.
The second step is checking the alignment of the tag. If a tag is not properly aligned with our buying partners, they will not receive the inventory requests to actually fill an impression and this is why a zone may have a very low or zero percent fill rate.
If each tag is aligned to demand as it should be, which your tags are, then we generally find implementation to be the main culprit of a zero percent fill rate. Are you currently using a basic text widget for your Sovrn tags? I have copied the link to a quick one-pager on tag implementation within a Wordpress site for your reference.