Could this be one of ebay's metrics where business sellers / high volume sellers are evaluated on percentages, but private or low volume sellers are evaluated over a rolling 12 month period and are only allowed a certain number within the last 12 months ?

The item stuck in the post might be the only one now but there might be more 'defects' spread over the past 11 months leading to the restriction.

If that's so then, if I remember correctly the restriction will be lifted as the months roll on, the earlier defects drop off and you return to Above Average?

I used to stop selling over Christmas and New Year  because as a low volume seller I was only allowed about 5 defects over the 12 month evaluation period and it seemed risky to collect several at Christmas.