Exposure to light (IIRC Bass comes in a brown bottle.)

Repeated cycles of hot and cold (don't store it in the trunk of your car.)

Is it possible that your beer store doesn't sell it often and you got an old batch? Another possibility is if you had it on tap: a proper bar set up with Guinness, Harp and Bass will use N₂O, not CO₂ for those beers; the flavor is much different.