Help needed.

i have created a video for college using various video clippings of a presentation and want to compile them together as one video. 

However- when I watch the video back some clippings are cutting across the previous- the audio at the beginning of one is cutting off the audio at the very end of another. I can't find an option to lengthen the previous frame so this doesn't happen and sentences are being cut off.

the videos are merging too fast basically.

can anyone help me please?

thanks in advance.