Sometimes in ResBook, the Rates and Minimum Stays of one of your stock units may change without warning, even though you have the correct settings within ResBook. 

However, rates and minimum stays in ResBook are superseded by rates and minimum stays that you may have entered in ResOnline. 

In order to fix this problem, login into your ResOnline account, go down to Setup and click on 'Rooms & Rates'

Once there, you will see your list on rooms on the left hand side. You will need to click on the listing with the yellow bullet point next to it.

Once you have click there, scroll down to the heading Year Round Defaults. Make all the fields in there blank. That will prevent ResOnline from overwriting the entries you have made in ResBook.

This will fix your problem of rates and minimum stays disappearing without warning.