S11 and Gain Relation

Hi everyone,

I have designed a compact dual-band monopole antenna. I considered the S11 and directivity as well as the shape of the pattern during the design process. I achieved two proper resonances at about 915 MHz and 2450 MHz. But now, I encountered a series of very low maximum gain values. I thought that if my S11 values are good (under -10 dB), then my maximum gain values will be good. But it seems that I was wrong. What may cause this very low gain values while my S11 values under -10 dB for both 915 MHz and 2450 MHz frequencies? I have attached the S11 and maximum gain values per frequency.

Thanks