Heater blowing cold air

As it worked right after you flushed it out but then stopped again check the temperature of both hoses when the engine is warm. I suspect you will find one hot one cold indicating there is still a blockage in the core. Not uncommon as the flushing will loosen scale but may not flush it out if there is not enough turbulence in the flushing stream. I would use a flush gun and reverse flush the system and the core. Then add a filter in the line to catch anything going to the core as well. I do that on systems with a lot of scale. If the problem is being caused by someone adding a sealer in the system the screen style filter does not work as well. For that a flushing machine that has a filter like the last picture will usually filter out the sealer material.