hey Mike
Would it be that you're not getting enough recoil power, hence the double feed and no semi auto action?
this could be due to the first spring you installed.
I also had this exact same behaviour when I tried to craft a hop-up rubber from a TM one, and I believe it's because a lot of gas was lost in the shot.
As I understand it, a stiffer hammer spring is going to have an influence on the valve opening duration, you're getting more gas out in the first fraction of second, when the hammer actually hits the valve button.
I could be completely wrong.