Sorry for the late response! I haven't checked for leaks yet but I can't hear any obvious ones. It's more like only so much gas will enter the mag and then stops (as expected), but there isn't sufficient gas to fully empty the mag, or if it does, it's then insufficient to lock back.
I've since tried the aap01 valves (they didn't fit so oh well), and I've had it suggested to try pulsing the gas in (multiple 1-2 second fills rather than one long one). I haven't had time to test recently, but will be trying all the methods suggested.
Thanks again for responding!
Edit: also, if anyone knows of a dual valve that will fit the we mags I'd be grateful as I prefer that style of filling and would mean consistency of technique with my mws mags