pull down your mag and post a picture and I can tell whether it's the spring, the follower or none of the above, pretty quick.
I'd like to know just for curiosity's sake.
I doubt Springfield is going to tell you how to convert a 10rd to a 13rd. That would be a liability should you be a CA resident claiming
to have left the (insert derogatory comment showing opinion of CA State Laws/regulations here) State of California.