This is a question that is...harder to answer. For me personally I've tried many different mediums for writing. I've done chat, Skype mostly, I've done chat sites as well (f-list), Rp in MMO's. The list continues, but I do find many of them just do not work for me personally. I find that, chat, as in skype and chat sites are...too quick for me. In other words mostly, the responses I get are boring and for lack of better words, simple. The response time is quicker obviously - and perhaps I enjoy more details and well thought out responses then what they can offer so I've backed out of all my rp on those sites.
As for the others - MMO's currently I have one partner that I'm writing with or will be - I'm playing Black Desert Online and its with a friend, an amazing friend that I've known for a very long time. We moved from World of Warcraft to Black Desert together, we chat on skype - and play BDO together, we both poke at one another to ensure we get the time we need or the other wants for the stories we have. So we help one another, in that - and its a MMO rp, a bit different then writing here I'm not certain it applies.
As for my stories here? I have all of them bookmarked, and I try to keep a running list in my story thread and O/o's so I can find what I owe easily enough in several places. Although I admit I recently found out I forgot one story. Somehow, I thought I had responded, and I hadn't. Now I'm struggling to find the words that I thought I had put to the story. So I'm trying to respond to that one. But that is how I keep track of my writings.
My question: How do you get over that hurdle, that moment in the story where all thoughts vanish. The story isn't over, and you still desire to write with that plot and partner -- just for that one post, that one response the words are dancing just out of reach for you? How do you get over that?