Re: suggestion of configuration for antenna of 5dbi
A couple things come to mind:
Some early generations of 1200s (some 1231) had the connections reversed between teh radio and the antenna terminals. So, even though the terminal is labled "Left," it was really wired as "Right."
Try connecting the antenna to the other terminal and check your signal strengths again. You could also change your settings on the AP to the other side ... either one should (move the antenna to the other terminal or change the setting to the other side) give you an idea.
If that's not it, then it takes us to a discussion of antenna radiation patterns.
As you increase the gain on the antenna, the radiation pattern stops looking like a fat donut and starts looking more like a cone.
What used to be touched by the donut is no longer touched by much of the cone (lower signal strength off the beam).
Try repositioning the antenna, try tipping it upside-down, try sideways. The only way to get passive gain from an antenna is to bend, fold, spindle, or mutilate the beam to concentrate more of it in a specific direction or plane.
If you didn't do a site survey to map all this out, you can spend a fortune trying to find the right antenna to do the job.
Certainly it's possible that one antenna, one AP is not enough to cover the area.
Without a survey, there is no definite way to tell.