Wi Fi problem

Hi. First of all I'm sorry if this thread is already posted, I really don't know how to describe my problem in a few words to try and search it.
Second of all, I'm from Argentina so forgive my English if it isn't great.

Last week I started to notice that mi iPhone 4 doesn't recognize all wi fi networks available in one area. Yesterday I asked a friend to bring his iPod touch and inside a shopping, his ipod showed 6 wi fi networks (my iphone none). My home wi fi network appears only when I'm ridiculously close to the modem.
I'm strarting to get really annoyed, because just 30 minutes ago, I called MacStation Argentina (kind of a official distribuitor here) and they told me that even if my international warranty hasn't expired, they provide technical service for all products except (guess....) iPhones.
Please tell me this ISN'T a hardware problem.
I tried restoring it from iTunes, restoring network setting (or something like that), and I am still having the same problems.
I appreciate ANY help, thanks a lot,


Juan