I'm not sure what you are doing but here is just a basic recipe that should always turn out.
6 tablespoons butter
5 tablespoons all-purpose flour
3 cups milk
Salt and pepper to taste
Pinch of nutmeg (optional)
Melt the butter in a medium sauce pan over medium heat. Stir in the flour, being sure to combine it with the butter very well, to make a roux. If the roux doesn't appear to be thick enough add 1/2 tsp more flour. If it seems too dry add a small amount of butter, 1/2 TBS or less. Let the roux cook, stirring constantly, for about 3 minutes. Do not let it darken. Gradually add the milk, about 1/2 cup at a time,
whisking or stirring vigorously to incorporate. Cook over medium-low heat, stirring often, until thickened and just starting to boil, about 15 to 20 minutes will be required. Add the salt, pepper and nutmeg.
By omitting the nutmeg you have a basic white sauce.
From here you can add your favorite cheeses (about 2 - 2 1/2 cups) - our favorite is Fontina and cheddar. Once cheeses are melted stir in cooked macaroni and eat it with child-like reckless abandon!