Issue with "User Presses a Key" Trigger

Hello! Being a relative Storyline neophyte, I would really appreciate help from some experts about a problem I am having. 

I have a slide with 4 text boxes. In each text box, the learner is instructed to press a key corresponding to the first letter of the concept discussed in the box. For example, in Box 1, the instruction reads, "Press the letter "a" on your keyboard."

I then have a trigger set up so that when the user presses the letter, the state of the corresponding text box is changed from "normal" to "selected". The "selected" state then reveals the title of the concept along with a brief description.

The problem I am having is that sometimes the triggers will work and sometimes they do not. Is it possible that a user who types the letters (I have identified on the slide) in another program (Word, Outlook, etc.) who then returns to my slide to then press the same letters is unable to do so? When I tried to replicate the problem this morning, nothing happened when I pressed the corresponding letters. I was still able, however, to click each box to reveal the content. But I'd rather use the "press key" trigger to reveal the content.

Any thoughts? 

(Please and thank you! in advance)

Lisa