Bandwidth counter not resetting!

For at least one (and possibly more) of my users, their bandwidth counter is not resetting at the end of the month, leading to their account being suspended at some point in the next month when they hit their limit. For example, one of my users with a 1gb limit:

May: 660mb
June: 340mb (up to their account being suspended today)

Obviously this is not so good. Why is it happening, and what can I do to fix it?

Thanks,

A second client site has just gone down for exactly the same reason.

Thanks,