how to add to a message thread in My eBay?

[firehorse*]

I rec'd a message from a seller   (2wks after my purchase of a pair of shoes saying she'd been hospitalised and would post that day, to which I replied wishing her well and looked forward to receiving them soon. She actually posted them 4 days later and the shoes are different sizes!!)   and I'd rather message her back on the same thread of the message in My eBay rather than creating a new one and can't see how to do that, though I'm sure I've done so before, so if you can help I'd be grateful.  

Thankyou, 

& Sorry, I didn't explain well enough! I can see a reply link on her 1st message, but to continue on the message where I'd already replied, I don't have that option, I only have the options to delete, archive or move to..