@ Carole - quickest way to resolve the issues - find a tech to adjust and set up your machine for the fabrics, threads and sewing you expect to do. There is no point in spending a moment longer putting up with niggles.
Some different things affect stitch formation for sewing and how they all work together plays a part in affecting the stitching performance. It's a bit like setting up a perfect marriage.
On each set-up there is a range or tolerance within which the machine can be set up at the factory. As long as the setting for XXX falls within the recommended range it passes the factory test. On a range of 1-3, an item on one machine may be set at 1.25 and on another at 2.75 but because both are within the permitted range they both pass. The performance of the machines will be slightly different.
The 12000 and 15000 and similar high end machines domestic machines are like any other highly engineered performance machine - they perform best after being fine tuned for the conditions and the user. Pianos are often re-tuned at least 3 times on the day of performance, twice before and again during the interval if necessary. High performance car engines are fine tuned for the weather conditions, load and road conditions.
I am fortunate to have found a tech who used to set up in industrial machines in a factory and worked mostly in the sampling room . When the factory closed he skilled up on domestic machines and now works for himself. I think the combination of his expertise, his attention to detail and the personal touch of being able to talk to the person who works on the machine makes a difference. He makes a good point about multi-purpose sewing machines which is to