Possible problems

LVC (Lost Vital Chunks)

If some chunks become unavailable, the alert lost_vital_chunks is triggered. That can happen, for example, if some Data Nodes are unavailable.

To see the list of unavailable chunks, use yt list //sys/lost_vital_chunks.

To figure out where the replicas of a chunk were, use yt get #chunk-id/@last_seen_replicas.

For erasure chunks, you could look at yt get #chunk_id/@stored_replicas to find out which parts of the chunk are still available.

Tablet Cell Bundle's health is not good

The UI on the Bundles page shows bundle states. If a bundle isn't in the Good state, you need to find out why.