Hi,
I would miss a meal so the pup is hungry, use a high value treat and indoors call pup feed treat by throwing away a few yards as soon as pup starts to eat treat recall again and repeat thowing treat again do this about 20 times, do this every day for 3 or 4 days. Over this time do not use the recall command anywhere else or at any time you think there is a possibility of failure.
Then practice in the garden for a few days before trying out in a park.
The important thing is to never call recall if there is a chance of failure untill it has been fully installed again.