You may have foil insulation in the residences which will stop the wireless.
15db antenna with 600mw should be good for about 15 miles. You could probably use 50mw and an 8db omni and things would be working better.
JP's right on.. your gain is WAY too high on the omni.. the verticle beamwidth is probably very narrow... the higher in gain the smaller your beamwidth gets.. Try a much smaller omni 5-8dbi and you'll get better results.
The other thing you have to consider is the TX power of the receiving device. Most laptops have a ~65mW wireless card, sometimes less. The card may be able to receive the signal very well, but its ability to be heard by the AP is unlikely due to the low transmit power.
Then you have other environmental variables to consider as well (ie: construction of buildings, antenna height and placement, trees or any other obstacles intruding on LOS). At the distances you're talking these are probably less of an issue.
You may end up needing multiple low powered AP's in a mesh type configuration depending on the environment, size of the coverage area, and power of the receiving device.
Power isn't everything! You could probably make this work with a 2-3 rb133's coupled with r52's and low gain omnis (5dB).