I've been working to increase my drug units, and a strange event has showed up. I'd get this message after bribing the FBN guy:

You seem to understand how this relationship works. I'll be sure to spread the word that you're good for business.

And, when I'd go to increase my units, I stay at the same units. Anything I could be doing wrong, or is this a bug?

You've tipped the FBN Agent enough, but you don't have the points built up for your next drug increase.