Whoa, I had exactly that same problem with the 1/4" foot I got with my brand new Janome. It was just plain flared out at the tip, and I couldn't bend it back, and if I moved the needle over a tick, the stitching line was too small. Totally irritating, so I switched to using a fat zigzag foot with the needle pushed over far as it goes. That and some tape gave me a consistent 1/4" PLUS the benefit of doing so with BOTH feed dogs grabbing the fabric.
I recently bought the Janome foot which has multiple metal attachments - one for 1/4, one for scant and one for separating fabrics to SID. That has a nice unbent 1/4" foot attachment and works great, but only the left feed dogs do any work.
Maybe this is a quirk of the Janome?