Arduino with ping sensor problem

I have a arduino duemilanov conected to a ping))) sensor. it is wired with the three core power cable. i installed this setup at 2 places. it worked for 1 room but not for the other. Both are using the exact setup but only worked for 1 place. Could environment be a problem? anybody has encountered a same situation? Both these rooms are new premises. the room that did not worked has just been refurnished whereas the room that worked was not fully furnished.

thanks for reading.

How long is the wire? Define "worked".

the cable is basically around 10meters long. worked as in it achieved what i wanted, displaying readings when i'm present and not displaying readings when im not present.

the problem is that in the room that is not working, be it whether someone is present or not, it continuously generates readings.