You have to be in the Arrangements module to look for new hotels.
Once you are here you will notice in the SERVICES or in the SUPPLIERS tab that the hotels you sourced before informed you that they are fully booked.
What you have to do now is to look for new hotels. You just have to select your segment as in the image above and hit the magnifying glass to source for new hotels. You will be taken to this page, where you can Search (green button) and Select (right hand box) as many hotels as you need.
When you finish your selection, click the orange button PROCEED. You do not have to worry about the hotels that have already answered. They won't get your request twice.