drain plug on 425 block part 2

Hey fellas, so you guys helped me locate the coolant drain plugs on my 425 a few months ago. today I finally decided to do a proper flush of the cooling system but when I unscrewed the drain plug, nothing came out. yes there is definitely water in the rad and its topped up. I am wondering if anyone else has experienced this? is it possible that enough rust has built up in the water jackets to clog the drain plug? I tried poking it with a rod and it didn't go that far in, it felt like something hard in there. I wonder if the previous owner plugged it up for some reason? I am very frustrated as I wanted to drain all the fluid and run thermocure to clean out all the crud in there.

 

I usually poke a screw driver in it and yes sometimes you might have to whack it with a hammer.

 

Thanks, that did it for me. taking a screwdriver and tapping it until the water gushes out! Thank you!