Why can't i turn my engine by hand?

I have a 2005 Ford Ranger with a 5 speed Automatic 5r55e 3.0 2wd. Stock transmission unit was giving me problems so i recently got a hold of a junkyard transmission for a good price with a low miles. I currently have the new(junkyard) tranny bolted to my block and also the 4 torque converter to flexplate nuts. While hassling with the 4 TC to Flexplate nuts i noticed that i was no longer able to turn the engine by hand. i popped out the spark plugs and still no luck. From previous maintenance, ive been able to use one of the crank pulley bolts as a "spinner" and my engine would easily turn over. Just for curiousity, i bolted up my stock starter and unplugged my coil,and to my surprise it turned over easily with theflick of the ignition, (this is how i ended up turning my engine over for the TC nut installation) ...
Now i have the 4 nuts in place and my exhaust bolted up, .this time, i left the coil ON, and started my engine, RUNS GREAT but i still cant turn my engine over by hand. why?