Ok so the walnut mask is described as a R50 synth. As the mechanic bluntly states, "There's your problem."

You're going to have a >20% fail rate even at 46. You will start to notice your "improving curve of difficulty" at about rank 47. Every synth level before you you've noticed no improvement in making them because they are too high for you still.

Carpentry is lucky to have a R50 synth grind. That means that your R47-R57 will be relatively easy.