It's possible that the signal for the flat screen is too strong on some channels causing distortion on other channels that are near by.  For example if you have channels 23,24,25 and they are all quite strong they can cause distortion on each other and none come in.  The 60 and 70 foot cable runs reduce the signal enough that they don't interfere with each other anymore, which is wny the longest cable run gives the most channels.  Here is something to try, move the living room TV to the bedroom with the most channels and see if it comes in better there, if it does then you have your answer.