I did a very strange thing yesterday and took my Blur round Glentress. It took me a while to get used to riding with rear suspension again - not helped by the fact that the Monarch was losing pressure as I was riding. The bike hasn't been ridden for over 6 months so I don't know if that's a actor. The shock is 4-years old and they've never had the best reputation anyway. Service/fix or replace with something else?
Take shock off bike and immerse in bowl of water, look for leaking air.
Then decide. Its probably the air can seals or possibly the schraeder valve leaking.
It ain't the valve core but I do wonder if it's that crap valve hinge design. I'm sure they issued replacements. I'll do the under-water trick to see what I can discover....
Mine was doing it that too, only not quite a quickly as yours seemed to be losing air. I sent it to loco and got new air seals, was a cheap job thankfully and feels just fine now.