Are you sure you got all the chemical flush and baking soda out of the whole system? I have seen problems like this persist because someone didn’t adequately rinse the system out. I would add the flush and fill kit to be sure the whole system gets both flushed and rinsed. This way you don’t need to disconnect any hoses and you can be sure you flush the whole system, not just the heater core. After all, you might have blockage somewhere else in the system that is causing your problem.