2000 Blazer Heat Problem

My son's 2000 Blazer has no heat at the beginning of every heating season. The return heater hose is always cooler. I back flush the core which fixes the problem for a few months. During the back flush the water runs clear immediately. I always let the motor run with the cap off, facing uphill to get the air out when I am done but the fix only lasts a few months. Any ideas would be appreciated.

the prob is the heatter core, i just blow air at low pis in the hose and that fixes it.

the prob is the heatter core, i just blow air at low pis in the hose and that fixes it.

Thanks for the tip, I'll give it a try