Before you push down the lever to lock your clamp, the "click lock" on the sliding jaw assembly must be engaged.
Follow the steps in the comic below to familarize yourself with this exclusive All-In-One feature. Then you'll be ready to try your clamp on a piece of scrap.

The 99" clamps have a spacer to keep the rod from wiggling. One spacer should be located between the Fixed Jaw and the Sliding Jaw, about mid-point on the rod. Loosen the set screw and move the spacer toward the Fixed Jaw to use the clamp for small workpieces and then return to the original mid-point position. If you find a second spacer between the Sliding Jaw and the End Cap, slide it to the end of the clamp and tighten the set screw. It shouldn't interfer with most applications. If you need just that much more distance between the jaws, the second spacer can be removed.
Please share your pictures (in jpeg format) and tips of your projects where your All-In-One was used.