pt-online-schema-change locking replication - troubleshoot

Running pr-online-schema-change on a slave to optimize a table. It is locking the table so replication is falling behind.
I have run this successfully on this server and others, but the lock is happening on the latest run.
What can I do to troubleshoot?

Server: Debian GNU/Linux 8.7 (jessie)
Mysql version: 5.5.54-38.7-log
pt-online-schema-change 3.0.12

command:
/usr/bin/pt-online-schema-change --execute --print --alter " ENGINE=MyISAM " u=$user,p=$passwd,D=$DB,t=$TBL --no-check-replication-filters --recursion-method=none --critical-load Threads_running=100

No slaves found. See --recursion-method if host db4-dfw.star2star.com has slaves.
Not checking slave lag because no slaves were found and --check-slave-lag was not specified.

2019-07-02T16:42:38 Copying approximately 3404389414 rows…
INSERT LOW_PRIORITY IGNORE INTO starbox_cdr._cdr_new (id, locationid, extensionid, accountcode, src, dst, dcontext, clid, channel, dstchannel, lastapp, lastdata, start, answer, end, duration, billsec, disposition, amaflags, src_display, dst_display, s2saccountcode, accid, cac, cac_display, pac, pac_display) SELECT id, locationid, extensionid, accountcode, src, dst, dcontext, clid, channel, dstchannel, lastapp, lastdata, start, answer, end, duration, billsec, disposition, amaflags, src_display, dst_display, s2saccountcode, accid, cac, cac_display, pac, pac_display FROM starbox_cdr.cdr FORCE INDEX(PRIMARY) WHERE ((id >= ?)) AND ((id <= ?)) LOCK IN SHARE MODE /pt-online-schema-change 23533 copy nibble/
SELECT /*!40001 SQL_NO_CACHE */ id FROM starbox_cdr.cdr FORCE INDEX(PRIMARY) WHERE ((id >= ?)) ORDER BY id LIMIT ?, 2 /next chunk boundary/

Id User Host/IP DB Time Cmd State Query


96625366 debian-sy localhost 0 Query show full processlist
96625523 archiver localhost starbox_cd 0 Query Sending INSERT LOW_PRIORITY IG --> pt-online-schema-change
96469822 system us star2star 33 Connec Waiting INSERT IGNORE INTO sta --> replication
96625524 archiver localhost starbox_cd 35 Sleep
75161925 system us 13366097 Connec Waiting