Sort(select( on a parent child table

[mclutter]

I have a pair of parent-child tables, and I'm hoping to figure out a way to auto-select in the second based on responses entered into the first. The normal sort select command is being pretty finnicky, and not correctly registering responses from the first table. Any help would be greatly appreciated. 

Thanks

[Steve]

Auto-select what? From what? What is "the normal sort select command"? In what way is it being "finnicky"?

Please provide a screenshot of the expression you're trying, and a screenshot of the configuration of the column in which the expression is being used.