I had to space some of my cameras down because of this issue with the soffit and gutter board. It doesn't take much for the IR to bounce back.
You should also move the tripwire up on the screen. This will get it into the gate opening in 3D, even if it does not look that way in 2D. I believe tripwires are tripped when 50% of a detected human has crossed it, so if you can get it to chest level, it will trigger sooner.
Also, if you are wanting to capture people coming through the gate, you may want to consider a B54IR-Z4E. You will get more zoom, more IR, and not be asking too much from one camera to cover the lot and the gate.