Open the temperature and pressure relief valve on the side near the top of the heater. Or it may be on the top.

If you are using the typical plastic outlet on the tank they do not have much clearance.

Even a small amount of scale will clog the drain.

Sounds to me like there's enough crud in there to block the passage once it starts flowing. Suggest the "coat hanger trick"....by which I mean running a stiff wire up through the open valve and wiggle it around to dislodge the obstructions. If there's that much crud in there, I suspect you may have to stay right there and keep wiggling the wire while it dribbles out annoyingly slowly.