Strange W98 defrag behavior

On one of the Windows 98 machines I have, when you choose to defragment the hard drive, the defrag program runs for awhile, then keeps starting over and over and over. Eventually it finishes, but it takes hours. No other apps are running that I’m aware of and no other machines do this. Does anyone know what’s going on and how to get the defrag program to run straight through?

TIA.