We have the Equalizer weight distribution hitch with anti-sway bars. When we first bought our TT and they were doing the walk through for setup, they showed us using the snap up lever to install the anti-sway bars. Then they said most people just raise the trailer using the electric tongue jack after placing it securely on the hitch ball instead of using the levers. This lifts the weight off of the sway bars and makes it easy to swing them into place, but it also provides a pretty good, granted not fool-proof, test that the coupler is securely fastened to the ball.
Agree with others, check for adjustment. Also agree about sway bars, I never tow without the sway bars firmly in place. The only other thing I can think of is somehow the coupler got closed on top of the ball. I've seen it happen where the coupler was closed or partially closed when it was lowered and the ball pushes the latch into the coupler then it gets closed without actually engaging the ball. The lift test would quickly identify this condition.