I started having problems with Mono about 8 years back. At that time it was only some of the darker colors but I started having problems with all of it, not shrinking even with A heat gun, sometimes not sticking well. Getting A good covering job and having it sag in the sun and then not shrinking back tight.
The answer was easy, just change to Ultra and never look back. I still have A lot of Mono out in the shop but gave up using it except for repairs on planes covered with it. The shrink rates are different between the two coverings.
If you like it keep using it, they have A lot better choice of colors then any of the other brands and I still like the shine.