Just to be sure: did you have selected "indoor" for the distance setting on the WA7210 (menu "Wireless -> Distance Setting")? This should be selected if used indoor.

Anyway, what you are experiencing might be the "hidden node" or "exposed node problem" (see wikipedia) or some kind of other interferences caused by too many APs for your office area. Or maybe, newer client devices in your office support newer WiFi modes and are causing interferences (read on for an explanation/solution).

Generally speaking, you can use the same ESSID (ESSID = AP group, BSSID = AP) with different channels. If you need to support 802.11b, choose 1, 6 and 11 or even better 1, 7 and 13 if available in your country) in circular fashion around bordering APs. If you just need to support 802.11g or n, you can use a step width of 4 channels, this means 1, 5, 9 and 13. If you use n or mixed b/g/n mode, be sure to select 20 MHz channel width. If you insist to use 40 MHz channel width, the step width between non-overlapping channels can be up to 8, so you have only two such channels (3 and 11).

But having all APs use the same channel is a guarantee for problems sooner or later, no matter what ESSID is used.

Also make sure that the APs (even on different channels) do not interfere with each other by reducing transmission power. To do this correctly, you should measure the WiFi signals in your office (I use the free NetSpot utility for measuring WiFi coverage), check for bad signal/noise ratio (SNR) and adjust the AP's power accordingly or even change the placement of the APs. This requires some time and effort, especially if you run speed tests - what you should -, but at the end it will work if carefully tuned, even with WA7210 set up indoor. :)