Thanks for reaching out—this is a common issue with full letter-size sheets.
A standard 8½ x 11 page doesn’t fit cleanly unless it’s folded in a way that matches the internal pocket width. The easiest method is to fold the sheet into thirds (like a tri-fold letter) rather than in half. That reduces the width and lets it slide in much more smoothly.
A couple of tips that also help:
Insert the folded edge first and slightly angle the paper as you slide it in
Make sure the sheet is fully creased so it doesn’t “spring” back open inside the holder
If needed, trim just a small margin off the width before folding for an even easier fit
There isn’t a special hidden mechanism—just a size constraint of the pocket—but the tri-fold method is the intended way to use full letter pages in that checklist size.