Is there always supposed to be a capital after a question mark?
There doesn't have to be a capital letter after a question mark. It depends on where in the sentence the question mark is.
In most cases, there is a capital letter after a question mark.
i.e. Are you sure you picked up my laundry? I checked inside your car, but it's not in there.
However, if the question mark is part of a quotation, there can be a capital letter after the question mark, but there doesn't have to be one. It depends on who is speaking.
i.e. "Don't you think we should just split the food between us?" Alex proposed.
"Wait a second ... where's Ben?" she worriedly asked.