New Heater Core Not Working

[Cars]

Hello! I just replaced the heater core on my 1999 Silverado 5.3. I have the heater box reinstalled and the heater core hooked up. Now I have no heat. I roughly tested the core before I installed it under the hood and it got hot. Both hoses are HOT. What could be wrong? Specifically, could I rule out anything that would prevent me from reassembling the dash? Thanks in advance.

[Silverado4x4]

_{Sounds to me you have it air locked, did you burp the air out of the system?}