My suggestion is to open the drain without turning off the valve on top of the heater, let the drain run with the heater under full pressure. The pressure will often push out the debris blocking the drain. Let it run for aprox 5 minutes until you're confident it'll drain on it's own without obstruction. You may have to open/close/open/close the drain a few times to jar loose some debris as it comes thru the drain valve. Once the drain is clear, turn off the valve at the top of the heater and open a few faucets to allow air in the heater while it drains. If you're daring, you could open the T&P valve but be warned....it may not close properly later and you may end up having to replace it.
Alternative: Get a 1x3/4" bushing (brass preferably but galvanized will work too) Install a 6" brass nipple in it, a full port 3/4" ball valve and a 3/4" i.p. X 3/4" hose thread adaptor. Turn off the water to the heater, pull/remove the lower heater element. Most of the water will stay in the heater under vacuum (think of lifting a straw from your drink with your thumb over the end of the straw). QUICKLY screw in the bushing/nipple/ballvalve adaptor you just built, with the valve in the "OPEN" position until you get the first thread or two started, then shut the valve and hand tighten the adaptor into the heater. You now have a means of draining the tank with a new, temporary drain (usually) above the height of the sediment.
Good luck.
