Was playing a LAN game just now with an Arcana Knight Joker fusion deck. I had King’s Knight on the field and Queen’s Knight in my hand. I attacked into an opponent’s stronger monster with Witch of the Black Forest from my field so it’d die and let me draw Jack’s Knight, at which point a polymerization I had set on the field some turns ago would let me fuse them for AKJ. This worked, but the poklymerization wouldn’t activate. Why is this? Earlier in the game, I tried to fuse AKJ with a Sealed Hex substitution, but it was my bad for not seeing AKJ specifically fuses only off the Knights there 
Also, we play non-errata GOAT format-era cards, so it oculdn’t be an errata Witch blocking Jack’s Knight from being fused the turn I drew him through her effect