This answered my question. When using cannister all howitzers could fire at close range. Some of them were not as effective as others.
But there was no "dead zone" in front of a howitzer.
My initial thoughts were that due to the nature of the piece it could not fire on a flat trajectory.
Thanks much to you for your concise yet detailed answer! (and thanks to the rest of you as well - I consider my question anwered)