I'm with Darth Muffin on this one. Just keep the mags loaded with 15 rounds for several days and then try to add the last round. I had the same problem with my JF mags and after time the last round would load. Good luck.
That's actually not a bad thing since it means you have good, strong mag-springs.
Just put a little more muscle into it, you'll be able to load the last round. With a little time and use, they will become much easier to load. Also, as others have said, I would keep them all fully loaded with 16 rounds for a while.
Another trick that might help is this. After you load 5 rounds, firmly rap the basepad onto the heel of your palm a few times to "settle" everything inside the mag. Then load another 5 rounds into the mag and again firmly rap the basepad onto the heel of your palm several times. Do this again at about round #14.
I've found this to help over the years in getting that very last round loaded into the magazine.
Yes the 16th round is a hard squeeze on my .40 Jet Funnel mags too. Make sure the rounds are stacking properly in the mag. As someone else said, leave it loaded with 15 rounds and the 16th should go in without much of a fight after it sits for a couple of days.
My Jet Funnel Mags in 9mm and 40 is hard to get the last round in. I tend to down load them one round when shooting at the range so its not a big deal but I guess its normal all 13 of my Jet Funnel Mags are that way.
What's odd is that I can "force" that last, 16th round into the jet funnel, but I can't insert it into the gun with the slide closed. Slide open, no prob, 16th round is chambered. Sounds like it maybe a stiff spring thing. I'm going to give it a couple of days.
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