So, this wasn't answered so I'll go over what I've noticed about the game (if I'm wrong, someone please correct me.)
The amount you train in openness is variable depending on their existing openness in the default trainings. A training that will give 2 points between 0-10 will start giving 3 at 22, and up to 5 or more at high openness. This is not unusual, this is actually how it generally works.
Now, for them to actually gain openness, I think they need to be 'away' for a bit, while not being in any other location for alone time. If they're low on happiness or stamina, however, they may be prioritizing going to the bar, going to the spa, going to the baths, or the gym, anything to get their mojo back. Doing this means they might not find the time to self-train.
Of course, all this assumes you've given them the dildo and just didn't forget. Happens, sometimes. 19 openness is what would happen if you trained three times manually after they hit 10 without a dildo.