I was sent a Smartsheet project and am not sure how it was constructed. The % complete column for the child tasks have the amounts entered. The parent task does not report the calculated rollup from the children.
I attempted to fix it by creating a new % completed column and entering the values by hand. When I entered the value for the first child as 100%, it rolled to the parent as 100%. Being that this was the only value entered this was correct.
Then I entered the second child value. The parent reset to 0%. Adding more children did not changed the fact that the value remained at zero (0%).