Wow, I'm genuinely kind of stunned that others aren't experiencing this problem. I'm not sure why it's just me.

I owned probably 20-30 Frixons over the years, purchased at various times, and nearly all of them had this problem.

The only constant (beside the human being using them) I can think of is that probably 75% of my writing was in Moleskine-brand journals, and maybe 25% on index cards.

Do you live in a warm climate or leave them in the car during summer?

Good question. It happened even with Frixions that never left my house (which is the majority of them) here in the northeast USA. 90F/33C+ days are not uncommon here in the summer, but the Frixion writing in my notebooks ever went "invisible", so I think we can rule out a heat issue.

The ink in most of the pens was still visible and hadn't gone "invisible", so I don't think it was working but laying down invisible ink. It felt like the balls in the ballpoints had locked up.

You ever try sticking them in the freezer to see if they just heated up?

No, never tried that.

The ink inside the pens could be clearly seen, so I don't think it was invisible. And the ball in the ballpoint clearly felt like it had locked up.