iMovie clip problems

When I insert a recorded video clip into my iMovie on my MacBook, it cuts part of it off. For example, the clip I want to insert is 5.1 seconds long and when I drag it into my movie, it cuts it down to 3.2 seconds which cuts off part of the dialogue. I have tried to stretch it back out to the 5.1 seconds, but it won't stretch. Why is it doing this? And is there a way to make it stop?