What Mac operating system are you using - El Capitan or other?
Let us go through the typical drill....
1. What are the properties of the import into the project - video or still or both? Brand/model/settings of camera that recorded the video might give us the information that we need for this part of the troubleshooting planning. In view of your problem, this is very important information.
2. What are you or the project setting as the project preset to match the properties of the import?
Please tell us what the readings are for Editing Mode, Timebase, Frame Size, and Pixel Aspect Ratio under Edit Menu/Project Settings/General.
Please give these readings even if the fields look grayed out.
The rest of the questions will dependent on your answers to the above.
Please consider and give more details. If any questions about the questions, please do not hesitate to ask.