hi! i'm anamaria, from chile, my english is not good, so i ask you to forgive my grammar mistakes...
i have a beatiful dobermann since january, he is four years old and he has been trainning so he is such a very good dog, but i have some problems when i take him for a walk, he sniffs a lot, so he stop walking and i pull his leash, i speak up, but nothing. what can i do?
desde ya muchas gracias,
anamaria

I'm gonna do my best to help here, and if this is wrong, I hope someone will correct me.
I used to have a Golden Retreiver that would get obsessed with sniffing something (usually something gross) on our walks. After I got my dobe, Sasha, on leash, I noticed the same things in her.
Usually just a good quick "check" with the collar and the heel or come command was sufficient, but every once in a while it wasn't.

If your dog isn't paying attention to your commands, he/she is placing higher priority on the object than than they are on you. Well, in my house, I am the big dog. So I casually walk over to the pup and get myself in between her and the object (this might be a good place to introduce the "leave it" command), as soon as she makes eye contact I give the heel command and start walking.

Sometimes I have to use a treat to break her concentration on the object, but I only give it to her after she has made eye contact and followed me away from the sniffable. This way she associates the treat with me, and not the idea of her sniffing a nice aromatic piece of poo.

That might be oversimplifying your problem, if so I apologize. If not, I hope it helps.

I've just noticed that with my Dobe, pulling on the collar doesn't work well, it almost needs to be a quick "pop" to get her attention.