There are some specific cases in which a clip will not add to certain target areas in specific templates. One such template that displays this issue is Ocotillo Desert Lines. In this template the 21st slide will not accept certain clips. You cannot add a clip to this target area if the clip is either 30 fps or 60 fps. There may potentially be other templates that have this issue. If this occurs, there are a few options to get around the problem:
Option #1: If you are working with 30 fps clips, just add a video clip to the target area immediately to the right of the problematic target area. Once you do that, you can then add a clip to the previous target area.
Option #2: You can add a 24 fps or 48 fps clip to the problematic target area. Once that is done you can then add a clip with any fps to this area.
Note: If you do not have a 24 or 48 fps clip you can re-convert one of your existing clips to one of these frame rates by clicking the Advanced Settings option in Step 1 - Import & Convert. Here you have the option to choose your frame rate, select either 23.98p or 48p. Either of these will work to correct the problem.