Two weeks is a long time. I say go ahead and send another message.
They may have their accounts set up the way mine is, which reminds me that I need to figure out how to change that! When I get a message on Rav there is a tiny little envelope in the far right corner. Most of the time it sits there a few days before I notice it. I think it may be the default setting, because I don't remember specifying that when I got my account.
I've also noticed that some people who use a lot of social media can become a bit nonchalant about returning messages, especially if they no longer have the yarn etc. These days people have hundreds of Facebook friends and tons of messages and emails and its easy to forget to send a message to someone you don't know. It's not rudeness on their part, just forgetfulness or busyness. If you send another message maybe that'll remind them they meant to message you back.
Craigslist etiquette is like that. No one seems to email back if they no longer have the item, or they are a bit terse when they do.
ETA: I have this problem with a yarn store that carries one of my favorite yarns. Seriously, they will go two or three weeks without responding to online order submissions or emails. I have to call them. But when I do, they are so nice and I get the feeling they aren't big internet people, which I understand. They do mail orders out quickly though, once I get them on the phone.