Keep in mind that not every mag is created equal. They aren't made with a CNC machine. They are stamped pieces of sheet metal, that get welded together and smoothed back out again. That's a pretty rough process. I'm sure they have a GO/NOGO gauge they have to pass, but theres still gonna be some give & take.
Yes I think MBX is made better than some, but that doesn't mean every single mag they make is exactly the same. I ordered 3 MBX mags to go with my HONCHO and 2 of them wouldn't drop free. But you just have to learn how to figure out where they're contacting the gun, and then make a small change to the mag where it is hitting. Many times it's not where u think it would be. I've found SV mags that drag on the very top left part of the mag where it contacts the left side of the frame in front of the mag release button, but MBX has already removed that portion of material.
It could also be another part that's being contacted, such as the magwell. I like to check how my mags fit in a gun without a magwell, then if they all drop nice and easy, put the magwell on and check again.
Also check the feed lips to be sure you didn't get an anomaly. I know people say there's no tuning needed on certain brand mags, but it doesn't hurt to check just to be sure. It also teaches you how to maintain your equipment better. Open shooters still should try to do a certain level of maintenance every so often. I check my feed lips at least twice a year.... At the beginning of the season & midway through. Mainly because I shoot allot, and abuse my equipment, and shoot quite a bit indoors so the hard concrete isn't very favorable to my mags.
Some years I forgot or neglected to do that and didn't realize until AFTER a match that my "gun issues" were actually my fault for not checking something, and learned the hard way.
But if you take a simple pair of digital calipers and measure the distance between the feed lips, looking at the mag from the back (bullets forward) I check the distance in 3 places ... The very back, the middle, and as far forward as possible. The numbers should fall somewhere between .355 & .365. (for 9mm ... I think .40 should be around .380 but I haven't shot .40 in awhile, so don't quote me on that) And they should at least be equal, or get WIDER as they go forward. You don't want them to get narrower. That just sort of makes sense. Also I take a small "fine" or "extra fine" diamond file and take a few seconds to just go over them lightly again to remove any burrs, or sharp corners/edges. Then lastly if you have access to a buffer, I like to polish the feed lips, just as a final precaution. Nothing extensive, in fact it prob only takes 15 seconds on each mag to get good smooth shiny feed lips.
It's just good insurance and ensures that the bullets come out Nice and easy.
If you don't have the Dawson mag tuning kit, or at least the video that comes with that kit, look for that. There's also some really good videos out there on YouTube that explain how to keep your mags working 100%.