I'm guessing that it's Hal 6's "ephemeral knowledge" routines blocking learning. Check out the table called "ephemeralDetect" (in the miscData folder, in the brain editor) - are these the words/subjects you're thinking of?
To get around it, either remove the relevant entries from that table, or edit the brain script so that it does not block learning. Preferably make a backup first just in case you want to restore previous functionality later (or in case something goes wrong!!).
Here's the script line that blocks learning when the "ephemeral" words are detected (it's somewhere near line 1700):
HalBrain.ReadOnlyMode = True 'Block additional file saves when ephemeral knowledge is detected
Either "comment-out" the line by putting an apostrophe ' before the line (the whole line should turn green in the script editor), or delete the line, and learning will no longer be blocked. Remember to restart Hal to ensure the updated script is used.
Hope this helps,
Grant