Can feel the flour in my cheese sauce

I have attempted to make nacho cheese sauce multiple times, and ever single time, I get the granular sort of feeling, like the flour hasn't fully incorporated. I start off with a roux, 2 tablespoons each of butter and flour. I melt the butter, then add the flour and whisked it for about 2 minutes. I then slowly add cold milk and continue whisking, adding a total of 1 1/2 cups of milk. I continuously whisk for several minutes, attempting to make sure there are no clumps of roux, then add my freshly grated cheese. I follow the recipe from binging with babish when he's making the nachos from the Good Place. Where am I going wrong?