Doyal,
An air shock shouldn't really loose too much pressure due to riding, albeit that you did do quite a long ride.
Having said that, I know that I've had the same rear shock (Fox RP23) for 5 years now, which I set to my required pressure....and I leave it. I maybe check it every 3-6 months or something.
It could loose pressure if you pump it up inside your house, then go out and ride in freezing UK temperature.
If I was you, I would pump it up again and see if it holds. If it is a new shock, it might just be bedding in.