Rub a few soapy bubbles over the muzzle end with the bolt closed, see if any bubbles grow, if so then its poppet. If you eliminate that then its not a valve leak internally.
Use the same bubble method mentioned on the gauge, check valve, between fill assembly and air tube, to eliminate all external possibilities before tearing it apart internally. Process of elimination!
Only 11 ways air can escape D:
1 Poppet leak
1 internal Fill Assembly oring
2 internal Gauge port oring(s)
1 internal Valve o-ring
1 external Gauge port oring
1 Gauge thread into adapter
1 Gauge adapter into gauge port
1 Gauge itself leaking
1 Check Valve
1 Check Valve Thread Into Fill Assembly
Could also be a sticking check valve (1-2 drops of silicone oil on next fill could help unstick, fix) @ fill assembly or the thread sealant that the check valve uses when screwed into the valve.
Most common in this order IMO are Poppet, Gauge, Gauge Oring, Gauge Port Orings, Check Valve, then the rest tied for last, unless you remove/install your valve a lot then you can knick it of course.
And of course, degas when attempting any repair on the leaks!
