15 days?! WOW! Me and my wife NEVER get more than 4 days out of any carto. The taste just gets "off"- not a burnt taste, just not the same flavor the juice has with a fresh carto.
But Hanzo said above- that carto is clearly burnt. I would agree with him that poorly primed or holes too small would be the likely culprits.
That being said, I do occasionally get a carto that goes dry frequently (I hate those). When they start to have a dry taste, I have to do some sucks without firing to get the filler adequately wet again. It really is a quality control issue with Boge's. Some cartos the filler is just way to tight, which doesn't allow the juice to soak in properly. Sometimes I will have one of these in a box, and sometimes every carto in the box will be like that. Again, poor QC on the part of Boge. But I'm sure they just have a machine pushing out thousands of cartos a day, with no real way to check each carto.