Too small gain of cpw-fed metamaterial antenna

Recently, I try to simulate a cpw-fed metamaterial antenna based on the paper below.
2_1.pdf
However, there are some problems on the radiation pattern. The gain is too small about -5.6 dBi.

The file below contains the model I made in HFSS.
new_1.zip
Can anyone tell why the gain is so small? The feature of metamaterial antenna? or the wrong setting of the excitation?