Can't fit damper back into suspension forks
Something is likely blocked or broken inside, and there's not much you can do it about it unless perhaps you contrive a way to open it up, which probably wouldn't be all that reasonable to attempt given the hassle and low ultimate odds of success. They're meant to be unserviceable in every way.

In a shop context the thing to do is call Suntour on the chance they either have a replacement (possible) or a fix (unlikely), or would warranty the fork  Their US office is helpful; don't know where you are though. If not that then replace the fork, which is usually where this goes one way or another.