I had this same problem. My vassal liked me, I had no troops of his raised, and the army tab showed that he had troops that I could raise. The raise levy button was greyed out, preventing me from accessing his troops. To me this was a pretty big deal, since this particular vassal controlled one third of the military of my kingdom (I kinda needed his cooperation).
This bug can be fixed, if you're comfortable with editing save files. Here's how I did it:
- Backup your latest save game, then open it up in a text editor (or preferably in a programming environment--to help you keep tract of the pairs of brackets that group stuff together)
- Find the vassal whose troops you can't raise. To do this, try searching for his first name. The longer name with titles and such won't be present in the save file, only his birth name. Check your vassals birth date to be sure you have the right person.
- In the saved data for this person, you'll find a section that looks like this: "raised_liege_troops={######}" That number in the brackets is the problem. The save file is saying you have raised his troops. If the troops really are not currently fielded, the brackets should be blank.
- Delete the number in the brackets and save your changes to the file.
This fixed the problem for me. Be careful, though. Be sure this is a glitch and not an oversight on your part. Haphazardly editing you save game can mess things up if done improperly.