I did the test succesfully twice for "Verbs: Infinitive 1" but it did not turn yellow.
And in my second try I received a full heart bonus. Despite that it is still not yellow. What could be the problem?
Have you already completed it, and are just trying to keep it at full strength? Sometime because of the amount of words in a section, it can take several attempts to bring it back to full strength (especially if you completed a lot of lessons the first time around-all those words will decay at the same time).
My suggestion would be to complete a couple of timed practices (if you haven't already bought the timed practice, you really should, it is completely worth it!). If that doesn't help, then maybe someone from DL can look into your vocab list to see if there is something else going on.