You can try contacting the retailer to see if its possible to exchange your item as it appears defective.
After 5 years of use the magazine catch on M9 KJ M9 finally wore out. I purchased the guarder one and as per Crunches response the fitment is horrible to the point that a fair bit of dremeling is required. The cheapest solution is to reverse the mag catch as stated above.
I would also like to suggest posting an ad in the parts wanted section as that is what I did when my mag catch wore out and a kind friendly user came to my rescue with brand new metal one.
"Its only a little bit on fire"