Phil, I have had similar results, and can't explain why. I currently take capsules now instead of liquid fish oil (and am taking a different brand due to expense). But, when I was taking liquid, I would occasionally have a bottle that would freeze up on me. In addition, here is another tip... if you store it in the freezer and finish up the bottle, set out the bottle on the counter for a few hours, and you will find that one more dose will appear! I assume that there is residual that forms on the walls of the bottle and stays there because of the cool/freezing temps. When allowed to come to room temp, it melts down and releases to the bottom of the bottle, and easily pours out!
So, don't throw out your bottle until you are sure that it is truly empty!