To be honest , this issue could be a couple different things.
I’m not familiar with MikroTik RouterOS. So send logs to Graylog not sure. Or are your referring to that you cannot see the logs on Graylog from MikroTik RouterOS?
If so, try using tcpdump on graylog. This should show if the logs are at least reaching Graylog server from MikroTik.
EDIT:
I just noticed this, So Im going to assume you unable to see the logs on the Web UI. Check the Date/Time on Graylog, ensure the timezone is correct.
Next, Ensure Elasticsearch is running and there are no issues you can also look into the log files for Graylog & Elasticsearch to find any clue on what’s going on. Posting you findings would help.