When we run a race and enter the bib # manually we want the oldest racer in the finish box. Sometimes it goes there after we finish the
last racer and sometimes it doesn't, we have to manually enter it in the finish box before he finishes. What are we doing wrong?

What program are you using?
In the menu there is a function to adjust the start order and the finish order automatically or manually, try different variations. Normally it works well in automatic mode and you only have to enter the bib numbers that are different like the DNS or DNF.