Sounds fun and boy did this make me miss the field work!
I had the issue with Linus on the dilly dallying on land, taking a whiz, etc. bringing the bird back. I thought through what the issue might be (along with my trainer). Sometimes it's stress, but why? Does he not want to give the bird back? Is he unsure exactly what I want of him?
So, I started doing formal recalls in the field (long, over hills, in shadows, etc.) and not allowing him to air off leash. I think this helped him understand my expectations in the field. It was familiar. He likes that! We would make it fun and put a dead duck in his path which he would scoop up on his way in.
If he was coming in with the duck, I let him hold onto it for awhile. This was especially important to him if it was the first retrieve AND a live flyer. On days when it was hot and we were running him far, I'd even let him lay down and hold it. He'd come in holding the bird and I'd just pet him and get excited over him and let him keep it for a bit. He'd come in and sit and I'd just release him without taking the bird. Or, I'd take the bird, praise him, and toss the bird for him as a reward.
It took about two weeks to fix the land retrieve and have him bringing the bird to me just as fast as he was in getting it.