Help please, mesh flattens when animating

Hello! I couldn’t really find solution to this problem and now my every model I weight painted has the same issue: mesh flattens when i try to animate it. It isnt that big of an issue if the movement is small from the default pose, but if I move for example, hand too far from the body, it behaves weirdly.

obraz

Does anyone know how to fix it? Sorry if it’s stupid question or if its repeated

Hi, this could be problem of the wights, topology, rigging.
Examples:
https://www.youtube.com/watch?v=EOlWWh8AwKMhttps://www.youtube.com/watch?v=H7ktSMgXMP8
https://www.youtube.com/watch?v=H7ktSMgXMP8

I will check these videos out, thank you!!