Hard to say what's going on there. Not only do my players engage with this, by the time the session is done, they've almost always each hit their limit of 4 per session earned and expended. I was thinking maybe it's the amount of content in one session (and thus more opportunities to earn and spend it), but it sounds like from other posts you're moving along at a pretty good clip. It could be the mechanism by which the Inspiration is claimed. For us, it's the click of a button and, usually, a hilarious gif accompanying it. At a table, particularly if space is tight, it might just get overlooked and forgotten.