You worked up time before distance. This could add another minute to the time. Then have the friend leave the dog and return to the dog randomly and feed while you go out of sight and return. The friend stays with the dog. It is also important that the dog relax.
Corrections only make the dog nervous and worried.
If you have a problem, you are looking at months to fix it. There are few probems worse to fix then out of sight ups and downs. This can be as often as every few seconds up to once during the sit and once during the down. Not because they're unwilling to do them, but because they get so anxious to do what you want them to that they anticipate everything. In these cases, if the dog is going to break a stay, he follows the handler when they walk away from him.