Hi Sheik,
Did you check if the missing alerts show in the SOI Console? You may take "nimid" value from UIM Console for these missing alerts and then add 'Source Alert ID' column to SOI Alert Console and then search with this "nimid" value.
If the alert doesn't show up in the console, check if this is happening for a specific pattern of alerts like
=> "Severity = Information or Minor"
=> Do they belong any specific probe
I would also suggest to go through USM Connector troubleshooting steps as mentioned at below page...
Troubleshooting CA UIM Connector - CA Service Operations Insight Connectors - CA Technologies Documentation
Let me know if this helps, if you still see any missing alerts after referring above wiki page, please raise a support case
Thanks
Brahma