Too thin
The sauce is watery. It coats nothing.
Fix: reduce it. Turn the heat to medium. Simmer uncovered. Stir occasionally so it does not catch. Most sauces tighten in five to ten minutes. A tomato sauce drops from a splash to a coat in that time.
If you need speed, a cornstarch slurry works — one teaspoon cornstarch stirred into one tablespoon cold water, poured into the sauce off heat, then returned to low heat for two minutes. Or a beurre manié: equal parts soft butter and flour kneaded together, whisked in a small piece at a time. But reduction concentrates flavour too. It does two jobs at once.
Too thick
The sauce seized up. It pulls from the pan in a lump.
Fix: add liquid. Use whatever built the sauce — stock, cream, wine, or water. Add it a tablespoon at a time over low heat, stirring constantly. Dumping it all in at once shocks the emulsion and breaks it.
For cream sauces: the liquid must be warm. Cold cream into a hot pan splits the fat.
One number to keep in mind
Temperature when adding liquid: low. Not medium-low. Low. The pan should barely steam. That gives you control. High heat means you are chasing it and losing.
