Shouldn't an antenna with a higher gain provide a stronger signal in the designed direction?
Directional antennas work well for Line of Sight applications (where both Tx and Rx antennas can see each other). If you are indoors it means you will have reflections which will create unpredictable hotspots in gain. My advice is to try moving the access point position to see the variations in received signal. Remember you may have to move it on the same scale as a wavelength at the frequency you are transmitting to see a significant change.
I can see directional antennas for WiFi working well in situations where you need to extend outdoor coverage (like needing WiFi access in a back garden) but not indoors.
Hope that helps.