Default WiFi problems on 4.4 stock i537

Hi everyone) I'am on stock official 4.4 update and I have problem with wifi (at&t i537 Active):
the phone finds some networks, but doesn't "see" others (including my home network, - and not only my, my friend's as well).
Tried to downgade to 4.3, but I couldn't even TURN it ON, because it just wouldn't switch to "ON".
Is there any way to fix the problem? (I guess it's a known samsung-software bug on i537)