Availability Group log_reuse_wait = Replication We have a SQL 2014 Availability Group with 3 replicas of our primary database - 2 in our main data center and 1 in our DR center.

We did a large index rebuild on the primary which obviously needs to replicate to the secondaries. I believe this replication has now finished. When I run:

select redo_queue_size, log_send_queue_size 
from sys.dm_hadr_database_replica_states 
where database_id = 28


I get:

redo_queue_size |log_send_queue_size
0           | 0
0           | 0
NULL        | NULL


Which suggests to me that the replication has finished. However, when I run 

select log_reuse_wait, log_reuse_wait_desc from sys.databases where database_id = 28


I get: 

log_reuse_wait  | log_reuse_wait_desc
6               | REPLICATION


I found this query:

select * 
from sys.dm_hadr_database_replica_cluster_states 
where database_name = DB_NAME()


which shows the state of the replication which again suggests that everything is replicated, but it can't be truncated for some reason:

is_failover_ready   | is_pending_secondary_suspend  | recovery_lsn              | truncation_lsn
1                   | 0                             | 4294967295429496729500001 | 793156000000001600001
1                   | 0                             | 4294967295429496729500001 | 793156000000001600001
1                   | 0                             | 4294967295429496729500001 | 793156000000001600001


The Transaction Log backup jobs are running and backing up as expected, but the % of the log file used continues to grow due to normal traffic. How do I get SQL to reuse the transaction log file?
It looks like the CDC Capture job had stopped running. Restarting that (which took a while) then cleared out the log and allowed it to rewrite.