pt-online-schema-change - same nb of rows in orig. and _new tables, but never stops

pt-online-schema-change ends up with the same number of rows in original and _new tables, but actually never stops, and looks like keeps on updating the _new table from the original one via triggers… The original intend was to run alter table <…> engine=InnoDB; so not sure why it can just not finish the job? Can anybody please advise? Thanks in advance…