If the yeast hasn’t frothed up, it’s likely it’s either expired or your milk was too hot and it killed the yeast. Check the expiration date on the yeast package. If the yeast is still good, your milk may have been too hot. You’ll want to warm your oat milk in the microwave (or on the stovetop) until it’s warm to the touch but not scalding. A good test is to dab a bit of the warm milk on your wrist. If it’s not scalding, it should be close to the right temperature.