I made some burton bridge from a kit and secondary-fermented it in a 25 litre keg with a tap and a cap capable of taking a CO2 cartridge holder (none was fitted) I left it in our warm utility room (circa 22°C) for five days to second-ferment with a standard amount of sugar then it was stored in the garage between 16-20°C (daytime). No leaks evident anywhere as the pressure was normal initially.
By the time the barrel was half empty the carbonation seemed less and the beer a bit flat, certainly different tasting to initial pours. I'm just taking a pint glass at a time rather than filling a jug.
Any advice on why this might be happening?