Issue
I'm making updates to mapped tables in Postgres but Heroku Connect is not syncing the changes to Salesforce. The changes appear in my _trigger_log, but the state is READONLY not PENDING or SYNCED or FAILED.
Resolution
When you see a READONLY state in the _trigger_log, it means that Heroku Connect detected a change to a mapping that is read-only. When this happens, the change is recorded as if it would be synced to Salesforce, but it is never synced.
To resolve this, you can modify the mapping to be read / write. That will allow future changes made in Postgres to be captured and synced to Salesforce. You can also update the records in the _trigger_log to be synchronized as if they were new by changing their state column from READONLY to NEW:
UPDATE salesforce._trigger_log SET state = 'NEW' WHERE state = 'READONLY' AND table_name = '<NEWLY READ/WRITE TABLE NAME>';