It also depends upon what condition the gun is in when your trying to drop the mag. If the slide is forward, you catch the mag safety and it puts a good deal of pressure on the mag and causes it to hang on the retention tab.
Try this,
Slide forward, mag release should be tight do to extra tension. You may have to apply pressure to the bottom of the mag to counter the spring tension.
Slide locked back, mag should easily fall from the pistol as your not fighting the mag disconnect/safety.
If, with the slide locked back, you have trouble getting the mag to drop free, you have an issue you need to look into. Either send it back to Ruger or follow one of several guides posted on here to remove, polish, lube and reassemble the mag release mechanism. It's basically two pieces and spring held in by a pin. Nothing to crazy.
On a side note, the new SR9c seems to have a D shaped mag release. I'm wondering how that will work out since the current setup twists the button a bit as it ramps up the lever. Might be not hand up when it twists. Can't wait to get ahold of one.