Evaluate Muzzle Technique. This sounds complicated, but it's very important. First, head over to Artspots or another such site, then find a ton of pictures involving a muzzle that you like. Don't just look at the muzzle, think about what was going through the artist's mind as they drew it......I'm really not getting through here, aren't I?
In short:
Sharp=Cool
Curved=Cute
This is all I think about while drawing muzzles.
Wwaaaaaait, you're stage/phase/level 3. Oh snap!
Perhaps you should ignore everything I just said. Or, it might be useful to you. Hmmmmm...gimme a minute.
You feel frustrated. You were previously able to do things correctly. Now, you perform tasks wrongly. Why? Has your actual drawing changed? Or is it only your PERCEPTION of your drawing? Unless you just lost your dominant hand in a horrible accident, it's probably mostly your perception.
I do have a suggestion other than copying the same head each time...copy different heads each time. As I said, find pictures of muzzles and copy them. However, make sure that it's not just from the same person, try to take a variety of art styles (that you like) into account.
Don't give up! Remember why you're doing this!
((If you are at all confused, feel free to reply to me. I would be ESPECIALLY happy to elaborate on the 4 Stages of Skill Mastery, if you have a spare hour or two.))