In my language there is a proverb which says often people take no use of what they know (especially their profession). E.g a potter (who creates mud jugs) never has a beautiful / good etc. jug himself.
I have found in some translated pages the following proverbs a long time ago. But I have no idea if they are natural to an AmE native speaker. If not, then please let me know how shall I indicate it in a normal language:
Shoe makers son runs barefoot.
The shoemaker’s wife, goes the worst shod.