Well if someone could tell me why my Colt Mustang Pocketlite .380 won't feed properly with a full magazine (6 rounds), but seems to work fine with 5 rounds in the mag, that might prevent me from trading it in on a Ruger LCP II this year.
That is pretty simple actually. It is the opposite of what Badger73 said.
It is probably somewhat hard for you to stuff that sixth round into the magazine. Once you get that last round in there, hold the magazine in your hand. If you push down on that top round, you probably cannot move it much. There is very likely almost no give whatsoever.
The problem is that the slide going backwards or forwards has to pass over that top round. The slide needs the top round to go down into the magazine a little bit. The "overstuffed" magazine DOES NOT GIVE and is therefore slowing your slide as it goes backwards or forwards inducing a malfunction.
Even if you load the gun at slidelock, your slide going forward has to strip the top round out of the magazine. If your fully loaded magazine is insanely tight, your recoil spring might not have enough strength to both strip the round out of the magazine AND push the gun into battery.
Also---if you try to load a full magazine into your gun while the slide is forward (i.e., in battery)........it is probably very hard to seat the magazine. In competition, I often see guys try to stuff magazines into their guns on the clock only to see them just fall back out because the magazine is too "full" to seat.
For all of these reasons......In competition or for home defense.....I often download one round for guns that manifest this problem (thankfully, most guns do not). A problem AR, for example, would only have 28 or 29 rounds in its magazine. I prize reliability and worry free reloads over the extra round or two. Many have made fun of me for this practice.....until they bobble a reload or induce a malfunction on their very first shot. Then they adopt my practice.
Being that this gun only has 6 rounds in it, you probably want all 6 rounds and don't want to download. You can cut a coil off of your magazine spring (springs are like $5 so if you ruin one no big deal, trim the bottom your magazine follower if that is what is binding, or just weaken the magazine spring through use. I have also seen gunsmiths file down the bottom of the slide where it passes over the bullet, but you have to be VERY careful as this can ruin the gun if done improperly.
You can also look for aftermarket magazine basepads that will give you a little more room in the magazine. Or if your factory basepad can accept a LITTLE dremelling to the inside bottom.....that might create more room in the magazine.
This is a very common issue. You need a little more space in your magazine and / or a weaker magazine spring.