Tripwires

I was trying to hook up tripwires with string, some in the air and some on the ground. They would not hook up. I also figured something else out. If there is no ground below the real "tripwire" block, it will not connect to string that is placed on the ground. Really, I do not know whether this has to do with the trap setup having to be completely mid-air, or completely on the ground. Thanks.