When your Tamagotchi is at/near its base weight, it'll refuse to play games.
Each character has a minimum weight. For example all babies have a minimum weight of 5g/5lbs, and all children have a minimum weight of 10g/10lbs.
Let's just say you have a Mizutamatchi (minimum weight 10g/10lbs). When your Mizutamatchi weighs 10g, 11g or 12g, it'll refuse to play games. But when it's heavier (13g or heavier), it will begin to play games.
I say just wait until your Tamagotchi actually needs food or only feed one snack at a time.