Open iTunes, browse to Library and then films. Right click the header bar where it says name, time, year, etc - select size from the list - compare this with the advertised size. If not consistent - go to Store on Main Menu and select check for available downloads. If this is not the problem- Delete the films from your ipads movie App - then connect to iTunes and resync again. Do the sync by cable for better reliability of data exchange.
Before going to the video App - double tap the home button and locate the App in the multitask bar, hold it till it wriggles, then tap the red'-' to close it - do a reboot as described by Diavonex.
Navigate to Settings|Video and change the setting of where to recommence playing from - if set to from beginning it does not matter - if set from where left off, change it, then reboot - go back afterwards and reset it to where left off again - open your Movie App and test your film.
On your PC, log into your Account under iTunes Store - (section=Home) and then click on the quicklink at right. Once in - click on Purchase history and view your purchases. There is a link to report a problem. If a film will not play, (or you have been incorrectly charged etc). There is always the odd chance that the server from which your film is downloaded from does not have a complete copy, or has a corrupt copy. Alerting the store to your problem should enable an alternative download for you. If they can do this then note it may be necessary to locate your media library folder on your PC and then delete the original damaged copy first. iTunes Store should be able to advise on the correct course of action.
I have had incomplete films myself once or twice - but usually the store detects it and automatically notifies that there is still a download pending when clicking on 'Check for downloads'.
Since your other films play alright - the logic seems to be that the film file is damaged in some way.
Best regards, Ct