sorry if this is long but i need help.
Merlin is 2 years old, we do conformation shows,agility,flyball,obedience, he is my first rottie but i don't know where i have gone wrong, the only way i can describe the problem is he is very dog orientated, he does not want to attack them he just only wants to play and greet them

, i don't mind this but when we are working it is like he pulls the shutters down on me, this is an example
placed in a recall position, and there are some owners with dogs waiting on their turn at the other side of the hall, i tell him to wait and leave him when i about turn to face him, he is watching the other dogs i call him the first time he is so focused on the other dogs he does not hear me so i will give a tug on the leader which brings him back to focus on me at the same time he gives a grumble

for being broken out of his trance.
next example
will be doing agility and as we are first there the trainer gives us extra time he flys around the agility course and hits all the contacts and listens to turns, then someone will turn up with a dog and he will leave the course and go to sniff the other dog,i can call him and call him but he just seems to shut down, i have to go and get him.
my obedience instructor says he is good enough for obedience competion but the problem of being to interested in other dogs is baffling us, we have tried making him hungry before obedience etc and feeding him in training,but this did not work he still switched off, tried toys, tried letting him mingle with other dogs before he worked this did not work,
where have i gone wrong,when he does switch off .