The button foot is great isn't it - ideal for shirt buttons and so quick.
Re the buttonhole foot - the short answer is, (as with all long feet) it is about keeping the buttonhole foot completely level throughout the stitching process.
just posting a summary of something I posted elsewhere about using a buttonhole foot.
It may or may not offer a clue about what might be happening with your buttonhole foot, but at least it may help you to suss out what will work for you.
Most machines with buttonhole functions (whether mechanical or computerised, 4 step or 1 step) can make decent and consistent buttonholes.
However, even the most whizzy gizmo TOL computerised machine with umpteen buttonhole styles plus stabiliser plates etc. will make rubbish buttonholes if the user doesn't stabilise, keep the foot level, adjust the settings, make a careful thread choice and test on like for like (lumpy) samples.
1. Stabilise the both the fabric and the long foot -
- fabric ... stabilise as needed to ensure a sufficient substrate for the stitches to form evenly and to bite and hold onto during wear and tear.
Use interfacing inside the fabric layers to make the fabric stable and on troublesome fabrics also use machine embroidery stabiliser (whichever type is suitable for the job) on the top, bottom or both.
- foot ... do what works to keep the long and wide foot completely level so that the fabric is moved correctly by the feed dogs from the start to end of the stitching. Sometimes that is to use a stabiliser plate, other times it can be to level up as you would with a hump-jumper approach. It's all about keeping that foot level so the feed dogs can work.
The issue with any long and wide foot, including a buttonhole foot, is to keep the foot absolutely flat during the whole process so that the feed dog has exactly the same amount of traction on the fabric throughout.
On some machines it can help to increase the presser foot pressure when the buttonhole foot is attached.
Steps and lumps from seam allowances or textured fabric (even small ones) can tip the foot slightly and as soon as that happens the feed dog looses it's grip or traction which is why the fabric will then stall and feed unevenly. As soon as the fabric movement stalls, the stitches are piled up into lumps. Often the key is how to control and compensate for the different levels in the fabric that tip the foot even slightly.
Also if the machine isn't a flat bed, lift and support the fabric around the machine with books etc. so there is no drag against the foot or feed dogs.
2. Test stitching -
Avoid the frustration trap of testing buttonholes on a flat sample of the fabric. If you are going to make buttonholes near seam allowances and edges then replicate the conditions on the test sample.
Stitching a buttonhole on a shirt placket and then stitching a buttonhole on a collar stand - the conditions are different so 2 different samples are needed so you know how to keep the foot level for the different conditions. Also you can work the buttonhole from different directions - toward the edge or away from it by switching around the way you present the fabric to the machine