OK I've worked out this is an interaction with the logic that the recipe will not be set if there is any item in the trash slot.

This occurs because ingredients are loaded in during the previous recipe, and when the recipe completes, some ingredients are left over and these end up in the trash slot ...

Items:
Decider combinator x 2
Buffer chest
Assembler

Connections:
Buffer --> Decider A input
Buffer --> Decider B input
Deciders A and B output --> Assembler (set recipe)

Decider logic:
A: Item A in buffer chest > Item B, output Item B = 1
B: Item B in buffer chest >= Item A, output Item A = 1 ...

Overall, I love this! But I also hate the names! I expect you will make them moddable though, right? I'd name them something like Crude/Low (and you only realize the quality wasn't even good before you unlock quality modules and see it), Normal/Nominal, High/Great, Exceptional/Superior and ...