Try constructing a URL of the form https://www.example.com/wallets/api3/notify/DEM/wallet/TXID where you replace example.com with your domain name, and TXID with an actual transaction id. Visit the URL in your browser. This should force the plugin to query the wallet about that transaction, and update the confirmations count.
Does this make the confirmation count update? Do you get back a JSON message?
If the above works, check your walletnotify line in your wallet’s config file. You should have placed a curl command in there. Does the curl command work from the host where your wallet runs? Could it be some networking issue?
Please try the above and let me know if you were not able to resolve the problem or if you have any questions.