If you are trying to split the signal from an Apple TV over number of screens you may find that even with HDMI boosters/amplifiers that when you switch the units on you have some screens working and some not.
This can happen even with the same cable run lengths !!
What you need to do to rectify this problem is very straight forward, just go into the Apple settings and change the screen resolution from Auto down to 720p. You may find that you can increase these settings through trial and error.