Invisible Symbols in reopened AI File

[Jiwon5FF9]

Hi there,

Some symbols are not visible when I saved the file and reopened it. When viewed in the Outline preview(commend+Y), the symbol is visible and exists in that position, but is not visible when viewed in Preview. If I double-click the symbol and enter the edit symbol and exit, the symbol is shown as previews. The symbol that disappeared always disappears when I reopen it even if it is remanded.
I'm using a lot of symbols to link each artwork, so it's so hard to click on some invisible symbols one by one.

Has anyone else experienced this problem, or does anybody have some advice as to how I can resolve it?