Things to look for:
Check the edges of the front of the holdopen and see if it is burred.
Check the notch on the bottom of the breechbolt that is caught by the holdopen and see if it has burrs.
remove the grips...(and with the weapon unloaded naturally) does the bolt lock back on an empty magazine? Does it do it consistently?
The holdopen action is a function of the pressure of the magazine loading button on the holdopen... check for freedom of movement of the holdopen.
Does the magazine drop free when the mag button is pushed and the grips have been removed? If it does, then either the inside of the grips are touching the magazine or your grip screws are too long and are touching the magazine when tightened down.
If these suggestions don't pan out, let me know and I'll open some memory files that haven't been used in years
and see if I can come up with some other reason.
Regarding the spongyness of the mag release button. Does it feel this way with the grips removed?