How could I improve my games' average playtime? (feedback)

So I’ve been making a game for about almost 2 weeks now and have been consistently sponsoring it and that’s going well but I’ve had players say the game is super fun, the game is very simple and im 100% sure theres no core loop breaking bugs, I just don’t know what feature im missing which would boost playtime or maybe it’s because there just isnt enough players in the server for them to want to stay long enough?

I’ve added a playtime boost, flags you can buy for the money you earn but that’s about it though… I didnt really want to just start making features that wouldnt help but I’d love to know anything you think I could implement as it would really help or just on the game in general

Heres the game

Heres the playtime analytics