The 5621 cannot be registered

Hi Forum Users,

There are three 5621s: A, B, and C. After data is configured on A, it can be found on the OLT that A is connected to B through 1:2 optical splitter and B is connected to C through 1:2 optical splitter. After three devices are connected, the OLT can detect device A but cannot detect device B and device C. If device B and device C are directly connected to device A, the device is connected to the OLT through the optical splitter. However, B and C cannot be detected after the optical cable is connected. This method has ruled out the optical splitter problem at point A.
After the optical cable at point B is directly connected to point C, the optical cable at point B cannot be found at point C. The optical attenuation from A to B and C is about -18. The optical attenuation is normal. Is it the optical splitter of B and C that has problems at the same time? However, the optical attenuation is normal, which is about -18.

What is the cause of this?

Thanks for your answers :)