[conundrum]

For your shoulder problem, I think there's a few possible solutions:

Try rubbing the joint with a clean cloth or (if possible) with warm, soapy water. This can get rid of any residual mold-release on the joint that can make it floppy.

If that doesn't work, you can try using some clear nail polish or nail harder to give the joint more friction. Super Glue is another alternative.

I've never owned a Figuart so I'm not entirely sure how to get to the joint but these methods have worked for me before on Transformer figures.