Yep, that can happen , you cannot make a seller respond to a message you send to them, but you can make them respond to a case.
So open the relevant case in the Resolution Centre.
https://resolutioncentre.ebay.co.uk
Enter the tracking number of the item sent back, or the 16 digit number on your proof of postage, enter the number with no gaps.
Escalate on day 4 if not refunded beforehand, eBay will step in, and force the refund.
Do not close the case early on any excuse the seller may give, and do not forget to escalate.
