Kids won't get in the car at pickup

This is my third week dealing with this. My boys (9 and 11) are dragging their feet, being super slow, suddenly needing to find stuff in their rooms right when I show up. Today I waited 20 minutes while my 9yo "looked for his iPad." Ex says she's not making them do this. Idk if it's normal adjustment stuff or what. Am I handling this wrong by just... waiting?