So similar to Mirodar's research with finesmithing, I noticed that if your cr.arts.design bonus is significantly higher than your crafts.materials.leatherworking bonus, your attempts to tool a pattern may be hindered. This may include buffing up your intelligence which would increase your design bonus at a higher rate than your leatherwork bonus, due to the different stat dependencies. I don't have any formal research on this, just what I've noticed over the past day or so, so would welcome any assistance. I found that increasing my design bonus just made things harder, until I made my leatherworking closely match it.
So my advice is work on both bonuses simultaneously and keep them very close together, or your design slightly lower than your leatherworking. With both bonuses around 240, I'm successfully getting the highest quality patterns on my most recent attempts.
Nasrin 28 June 2023