We have no access to your account so cannot see your seller dashboard, but you can, so does this give you any clues? If you have fallen below standard for selling, you cannot promote your items.
If you feel all is OK there, and it certainly looks like there is regular selling activity, and your star ratings look good, so nothing glaringly obvious for us to see here.
Perhaps contact eBay Customer Services, they do have access to your account, and can perhaps explain this issue.
To Contact Customer Services it's a Live Chat, or you can select to speak and request a Call Back. Click on this link:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
On the left sidebar you can see Have us Call you or Chat with Our Automated Assistant.
On weekdays lines open between 8 a.m. and 10 .m.
On weekends they open between 9 a.m. and 6 p.m.
@big_clarky