When crafting the duck in the sweatshop, most of the time it does not create the protoduck to launch. Sometimes it does, but usually it doesn't. I cannot seem to get a consistent reason on why it would not create the duck. I tried creating a brand new sweatshop and putting in everything in manually and the protoduck is created but then when I do it again - empty item again.

Am I missing something?

If you have more than 250 ingredients, the sweatshop cycles through batches of ingredients, creating a Rubber Ducky Part, before the whole Rubber Ducky is created. Could it be that?
If not, feel free to send me your save file and I can take a look.

It's base game with no new items so unfortunately it's not that. There is less than 250 ingredients and only makes one rubber ducky part which rarely makes a part (I've tried multiple times in multiple ways)
I can send you my save file no problem, but uh... it's one hell of a sushi belt but I have saved right next to the sweatshop that needs only a spider. Where can I send it? I can be found on Discord under the same name if that helps.

Possibly unrelated, the inserters seem to really be keen to put uranium 238 in the rocket inventory but nothing else. I've tried turning all inserters away but even then it doesn't really work.

Just for fun, I downloaded the new update and 2/2 ducks made. I'll be seriously playing tonight and let you know if for certain it fixed it. But it looks good so far.

Hmm, problem still persists in live environment. Works fine for testing then breaks for no reason I can see.