I have made various white sauce recipes, gathered from the internet. Never had any issues.
However, researching a little, this is what I found right off. Just a possibility, probably among many:
"Why does white sauce separate?
What causes a white sauce to separate? Is there a way to correct it?
Submitted by marymonis
A white sauce will separate if there is not enough added thickener (usually flour or cornstarch) or if it is not heated long enough for the flour to thicken the sauce (it should be cooked and stirred until bubbly, then 1 to 2 minutes more). If a white sauce is separated, try cooking it until bubbly. If it is still not a smooth, thickened sauce, stir together a little more flour or cornstarch with cold water, then add to sauce and cook and stir until bubbly. Continue the process until desired thickness. If you get lumps in a white sauce, remove immediately and whisk. If this does not help, strain out the lumps before serving."http://www.bhg.com/advice/food/cooking-tips/white-sauce/
I advise trying other recipes if yours is not working, although many of the recipes are pretty similar. Follow the directions closely when first trying recipes. Try not to improvise right off, if possible.