Dropping in one country while rising in the others.

I have a site thats working well, made some links here, some there, bought some guest posting...

All my keywords were rising but suddenly my main keyword is sinking in one important particular country (.arg). In other regions still page 1.
2 weeks ago was page 1 position #3 and now its on page 4 while in the other countrys still perfect. What happened?

Should I focus in build some links .com.ar?

Thank you.

If the site already have the targeted country extension, this is just an update bounce. If you are working with a Neutral extension like .com, net, you can use Webmaster Tools to tell Google which country you want to target. Did I miss something?

Thank you @seohug , the domain is .com and its targeted to all spanish people around the world, but argentina is one of the best targets for my site.

The majority of linkbuilding I did is .com, .net and 1 .com.co, thats why I was asking if was good idea try to focus on get some .com.ar!