13.8% day 1 to 1.4% day 7 does read like people get one return visit and then don't find a reason for a third, so the gap between those two sessions seems like the place to look.
Do you know what a returning player sees in their first minute back, like an offline earnings summary or a new unlock waiting? And are your day 1 returners mostly people who reached a particular upgrade or milestone in session one?