Hi,
The LDR is it inside the arch you have built, the arch is made out of what.
Is it fully opaque to light
I think you have ambient light problems, the LDR is not just sensitive to the LED but to a major part of the ambient light spectrum.
What colour is your LED and if its a normal LED it will not be emitting a beam of light, which is what you need.
Do you have the LDR and LED in light tubes?
Can you post closer pictures of your LDR and LED assemblies please?
Check your readings in a darkened room.
Tom..... 