It looks from the PIR detection drawing, which you can see if you check out the link below, as if adolfo2 is correct. It certainly appears to have the same detection angular range, both vertically and horizontally. What is clear is that detection is favoured when moving across the sensor, as opposed to to and from it, and this ties in with my experience, and the reports of many others, that you can find in this forum. A car can even sneak right up on it when moving directly towards it.
The trick to overcome that is to mount the camera such that any intruder will be walking across the frame, but in your case, from how you’ve now described it, that may be difficult. Horizontal or vertical, they will still be coming towards it, or away from it.
Angling the camera, so that what you want to see is more to one side, may help, and as long as it’s still in the field of view of the camera, it won’t really matter if it’s to one side or not. I guess you’ll just have to experiment. Tilting it further down might even have the effect of making an approaching intruder track across the PIR sensor, from top to bottom, and may well do the trick.