Do more tickets become available?

Looking at your listings from Shenzhen to Xiamen eight days from now, many of the trains are listed as being sold out. Does this mean that they are sold out to everyone, or just online? If I went into a station for example, might there still be tickets available?
And is it possible that more tickets will soon be available online on this site, because currently the train I wish to travel on is listed as sold out.