5.3 coolant won't flow, help!

[OKGMC4]

The upper hose gets hot all the way into the top of the radiator, but the bottom stays ice cold. I thought maybe a new tstat would help but it turns out the old one may not have been stuck as the new one does the same thing. I have an aftermarket compressor but have double checked my belt routing and all looks good, pump turns counter clockwise. It's full of coolant and the hoses pressure up. I let it get to 210 dgrees it just doesn't open up and flow. Any ideas from experienced folks? Thanks.