Remember the transmiter outputs a wave signal, so, it needs an aerial standing verticle. If you expect it to go into the tip of the aerial then it won't, put it in a tube and it will help.
Also, the transmitter, make sure that the rubber aerial tip points upto the sky allowing the side to point at the car since the signal comes out of the sides more than the tip (i.e. if you point the aerial at the car end on, its not got as much chance of getting a signal).
Its the same theory as wireless networks, I'm just useless at explaining it.
Edit: Not the best diagram but might help. Red is the waves leaving the aerial, reverse for the car aerial:
And yes, it is the WORST image ever, but might help