Because you're referring to a random thing, you don't know the actual problem. If you'd say "this problem is not yours", it'd be эта проблема не твоя, because you're stressing: THIS is not your problem (but something else might be).
You use Ваша If you are talking to someone you dont know or someone older than you. Its more formal and respectful. Just like 'You' has its formal and informal addressing. So; Вы (you -formal)
Ты (you- informal)