How can I tell why my 10-inch adjustable wrench is slipping?

Inspect the jaw faces for burrs, dents, or shiny raised spots. A single high point can prevent full contact, so the jaws may look parallel and the adjustment threads may feel tight yet still slip under load, especially on a partly rounded nut.

For anyone checking a similar wrench, clean the faces and inspect them under good light, then compare the contact on an undamaged 3/4-inch nut. Don’t file the jaws aggressively: changing their angle can make the fit worse. If there’s a clear damaged spot, or the wrench still slips on a clean nut, replacement is the safer option.