Cat 613c transmission issue

I just traded some work for a 1991 Cat 613c scraper. It's a really good running machine. The only issue is that it will shift into first and second gear and pull fine. When I shift it into third fourth or fifth the transmission doesn't engage. It shifts into sixth gear just fine and has plenty of power. It also goes into reverse with no issues. I checked the Trans oil and I don't see any metal. Any ideas of where to start would be appreciated.