The AWS.DynamoDBStreams.describeStream() function returns max. 100 shards. If the stream consists of more shards than that, the response contains a LastEvaluatedShardId value, which can be passed in a subsequent describeStream() call using the ExclusiveStartShardId parameter, to fetch the remaining shards.

Because the client does not fetch all the shards, it will miss records on very active streams.