According some of the other forums, Unisa are experiencing a problem with not enough people to mark the assignments this year.
Everywhere you get complains about assignments not being marked within a reasonal (for us) period.
My own solution is, do all the assignments (most have a 1 week automatic extention period) and make certain that I definately get enough marks to qualify for the exams.
The one advantage is that I now need to do the last assignments which I normally wouldn't need to do. This forces me to go through all my work, doing some practical stuff and then the preparation for the exams would be much easier and less stressfull.
It's a hassle trying to fit everything in, but at least I'll benefit in the end.
Good luck!