Switching Hub Problems

Please teach me.The company has the following network (SH: switching hub).We've been running the same configuration for about five years, and the configuration hasn't changed.The number of terminals may be increasing.

optical frets
|
Router (RTX1210)
|(A)|(B)
SH SH
|<Approximately 10 branches>      　　　　 |<8 branches>
Terminal (approximately 20 units), SH, wireless terminal (approximately 20 units), SH, wireless device

There is nothing wrong with (A), but (B) suddenly only connects two of the eight branches, and after a day, all eight branches are no longer connected.I replaced SH, but only 2-3 branches are connected.Any of the eight branches is fine, so if you skip 2-3 branches, everything else will be connected.You can't find any regularity about where to skip and where to connect.Not only certain branches are disconnected.

I don't think it's a hardware problem and I don't know where to fix it.Please let me know if you notice anything.