I had a Brunton bubble problem probably 20 years ago, at that time they told me that it was caused by going up in altitude, and that by going down the bubble would go away. I had never had it higher (at that time) than maybe 5000 ft, and was at about 100 ft when talking to them, bubble still there. You will notice that in neither case did they actually offer to do anything about the problem, like offer a replacement compass...