A few factors can be the cause of you freezing in your sleeping bag, even though it’s made for the required temperature. Sleeping bags insulate by trapping air in the fibers/down filling, this air is warmed up by your own body heat. Having too big a sleeping bag will increase the amount of air having to be warmed up, which can make it seem colder. Conversely, too small a sleeping bag will also give a colder experience (compressing the filling).  A few things can be done to maximize the performance:  Make sure to put out your sleeping bag at least 30 mins before you plan on using it. This way the fibers/down aren’t compressed and have a chance to puff up, making them able to trap more air. Do not move too much in the bag.  Do a few exercises to get your blood flowing before going inside the sleeping bag. (Not so much that you start sweating, as this will have a negative effect).  Warm up the sleeping bag before going to bed by putting it around you as a blanket.