Peephole cam not detecting motion

My brand new peephole cam will randomly alert me of motion when there is none, or if I open the door, but will not detect any actual people walking up to my door, on my porch, putting mail in my mailbox; the actual reason I bought it. Literature says screen/glass door "may affect" motion detecting, but does that mean it will not work at all? I took off the screen and now have just a clear glass storm door in front, but it does not detect motion unless I open the storm door and get about an inch from it. Is this normal? Or is there something wrong with it?