What size on disk does the larger clip have? What codec are you using? Maybe what is happening is that it is very large and you need to wait a bit for it to play, or maybe try the VLC player to find out if the codec is the problem.
Try rendering the project to an image sequence. If that is OK then use the image sequence to render the deliverable file and make sure that you follow the client's exact rendering specifications. This is not a frame size that traditional compression software supports natively.
I answered in more detail in a similar thread that I think you posted earlier.