I'm a postgraduate student, and I was allocated a very mixed flat in my private halls. One of my flatmates is a first year, another is doing a PGCE, and all the others are third years. We are all doing very different courses, so have vastly different schedules. We are also all in different 'places' in our lives (age-wise and in terms of interests, commitment to education etc.), so we don't naturally gel. We don't 'not get along' or anything like that, we just don't see that much of each other. We chat when we happen to be the kitchen at the same time, but otherwise I spend most of my time with people on my course, as I am more similar to them in terms of personality and interests.
Don't feel down about it, most of the time it's just the luck of the draw who you get put with. Some feel like their flatmates are an extension of their family, others hate their flatmates with a passion. If you don't have any complaints about your flatmates, and not hanging out with them is the biggest worry you've got, you're luckier than most!