[ Mass Effect 1 ] I still have not gotten the follow up mission?

Yeah that seems like a weird bug.

How the assignments are supposed to go:
1. Become a Spectre.
2. Talk to Kahoku. He will tell you about his missing team.
3. Go to Edolus to find the team. kill the Thresher Maw. You may also need to click on something there (like a dead body or beacon)?
4. Report back to Kahoku and tell him you found his men. He will tell you he will contact you later if something else comes up. He will then leave the Citadel.
5. He will contact you when you access the galaxy map. This will open up the next cluster (with the planet Binthu).

So this assignment may be bugged in your playthrough for some reason. No idea why.

Check if Kahoku is still on the Citadel. If he is, make sure you tell him about his missing men.

I cannot remember if Kahoku calls you when you first access the galaxy map next, or if he calls you after your travel to a new location.
- If it is the former, then you should have already had the call. NOTE that another call may have overridden his call. For example, Hackett may tell you first about peoples reaction to your Al Jilani interview you did just did the interview beforehand.
- If it is the latter, then some other calls may take precedence; for example, Hackett tends to call you for some assignments when you enter a new cluster or system. You may need to travel to a previously-visited cluster or system to get the Kohaku call to drop.

That is all I can think of. Sorry if I cannot be of more help.