This is normal. The first video clip that you add to a project determines the frame rate allowed in future clips. For example, if you add a clip that is 29.97 frames per second (fps) as the first clip in the project, you will not be able to add a clip that is 15 fps.
You can resolve this by converting the clip that has a different frame rate to match the rest of the project. Start a new project and add the clip(s) you need to convert to the Timeline. Go to Make Movie and select AVI. Click settings and look at the Video settings section. You will see a pull-down box for Frames/second. Adjust this setting to match the rest of the original project. Click OK to exit settings and click the Create AVI file button to create a new file. Now you can add this video clip to the original project.