GMRS Repeater Antenna question

I have set up a repeater for GMRS on a hill overlooking the valley that I live in. I am using a Motorola RKR-1225 with a good set of cans.

My initial antenna choice does not seem to be giving me the coverage I was expecting. The elevation of the hill is roughly 400’ and I am using a Workman UVS-300 on a 20’ tower. It seems that I am getting great distance performance through the repeater, but not very good local coverage. I am wondering if the ground plane on the antenna is causing an umbrella affect, or maybe I am in the shadow of the hill.

Either way, I am considering upgrading to an Andrew DB408-B. It is a 4 bay dual radiator exposed dipole antenna which boasts omni-directional TX/RX pattern. I have heard great things about this antenna, but would like to hear any comments or ideas before investing another $600.

I guess I am slightly foolish for just assuming that the UVS-300 would perform well on the hill. I should have obtained a band specific antenna instead of a dual band amateur antenna for this project.

All comments are welcome, as I am looking to optimize the first GMRS repeater in our area.