Two hard flints never grind well.
More German Proverbs:
If the wolf would cease his running, the people would cease their shouting.German
Don't let your tongue say what your head may have to pay for.
German
One cannot wash a blackamoor white.
German
Make the cap fit the head.
German
He who makes himself honey, will be eaten by the flies.
German
Let your purse be your master.
German