There could be several reasons for this - but we need some more information before we start making suggestions.
Can you post a screen grab of your Wireless and the Advanced tab when in advanced mode?
There's most likely some interference near that customer
Here are A few quick suggestions:
- Telnet into their radio (don't use mac-telnet, use real telnet it's more robust) - then do a scan:
interface wireless scan 0 (or whatever card it is) duration=5 - see what you pick up - if you're using 5 or 10mhz channels - switch the AP over to 20mhz so you can see if what's going on in that band too.
If there are lots of AP's on that channel - pick a different one.
Try limiting the maximum negotiation rate - that is if the max download/upload you have your customers set to is 1 MB, there's no reason to have them negotiating at 54mbs - This will improve the quality and reduce the frequency of disconnects. Setting the rates can be tricky though and if you're not using all MikroTik and set the rates exactly in a certain way - some devices may not connect.
It could be a fresnel zone issue - even though you're signal is relatively good - a blocked freznel zone will result in a poor quality link - Try moving the antenna up, down, all around.
Let us know what you find out
Hope that helps!