For one of my dcbs it seemed to just be a unconscious thing to put things in his mouth if he wasn't actively playing with them. For him I started making him out the toy he mouthed in the sink or dirty toy bucket, and he lost it until I got around to cleaning it. He figured it out, but still chews on his fingers... Yuck.

15 months might be a bit young though you might just have to continue with reminders until they're able to connect losing toys with putting them in the mouth.