I've noticed on mine that it barely holds the locked position. Pulling an empty little utility trailer will have mine not locking. And in threads by others, it seems common that way.

Makes me wonder if you've got something going on that's imparting a bit of drag that's causing you to not lock and burn more gas. Could be as simple as low tire pressure, or a dragging e-brake.

'97 T-100 SR5
'86 Toyota's, the variety pack (all gone)