So you are saying that you are actually swapping the laptop screens over - not a job for the inexperienced, so either you know what you are doing or more worryingly you dont.
If its the latter then you could have stretched the cable that connects the screen to the base unit, and if thats the case you better return it to DELL if you have any warranty.
Regardless of which way you look at your post - External monitors work, and your original one and spare one don't.