I had to end up using Odin to flash the latest update. I never even received a single update from the initial release. Just make sure you flash the CSC file and not the Home_CSC file. Backup your Galaxy using smart switch before doing this.
My phone was saying that it was already up to date. But I know that there was that new update on 01/31/2019. So I talked to Bixby and asked if there were any updates. The new update came up. I was able to update my S9+ to the latest version and didn't have any problems updating. Hope this works for you.
I tried asking Bixby and Bixby said searching and it showed that I'm on the latest update from 1st of August (something like that), other people I know and my sister and dad already got the update except me? Please help?