Hey Andrea

It's said that TL-WA7210N with Serial number 137xxxxxxxxx has some problem in RF Output Power.
See #7 in http://forum.tp-link.com/showthread.php?59935-TL-WA7210N-Channel-bandwidth-and-rate&p=97348&viewfull=1#post97348

So what is the S/N of your unit?

Besides, you may also check the following things:

1) Are the two 7210 tied up tight? Wind can sway the devices. If the two unit are far apart (like 2km ), a small angle alteration can cause huge signal loss.

2) Have you tried to reboot the devices?

3) Set another static channel to avoid channel interference.