No, not nana-bitty

I made cheese sauce from scratch tonight, and it went kinda bitty, while still liquid. Does that make sense? Not lumpy, but it was like something hadn't dissolved properly. Here's the recipe I followed (to the letter, I was paranoid about messing it up!)

50g butter (i used marg, didn't have butter)
2 tbsp plain flour
2 cups milk
1 cup grated cheddar cheese

Heat butter in saucepan over medium heat for 2 mins or until melted and foaming.
Add flour.
Cook, stirring with wooden spoon, for 1 min or until mixture bubbles. Remove from heat.
Slowly add milk, 1/4 cup at a time, stirring constantly to prevent lumps forming.
Return pan to medium heat.
Cook, stirring, for 5 minutes or until sauce boils and thickens. Stir in cheese until smooth, remove from heat.
I followed it all exactly, except after the milk went in it probably took 10 minutes to thicken and boil, not 5. I wasn't timing it, but it sure felt like longer!

Any thoughts on what might've gone wrong? Or does someone have a better recipe?