Problems with Wlan

Hi, I got a business with free Wlan. We had a guy that installed everything but we always had problems with it. (3 year) Ethernet works great but the Wlan has never really worked. The problem is that it keeps throwing people out all the time. Every 12min. He says that we have to upgrade the hardware again, just after 3 years. I think hi did something wrong with the settings since it is always every 12min. This doesn't happen to everybody but I would say 70% of our customers. What do you think? I just need to be able to tell him something since i'm really bad at this stuff.

We have one connection to the Adsl modem (running on copper), one firewall that sends it out to 6 wlan antennas or what it is called.