Probably because of how your filters are set. The Resource List has a little arrow at the right. Click the arrow or anywhere in the top panel to open your filters. This is how you show only the resources that you want to see.
Note that there is a Scheduled filter, and it is probably set to Unscheduled. This means that when you schedule a resource, it's removed from the list. You can confirm this by dragging a resource to the job grid. It will disappear from the Resource List. Drag the allocation to the trash, and the resource will reappear.
If you want to see both Scheduled and Unscheduled resources in the Resource List, set the filter to All and click Apply.
Most screens in ControlBoard have a similar filter list, and they all work the same way. If you're not seeing something you expect to see, the filters are the first place to look.