gain of a rectangular microstrip Dipole antenna

Hello,
I am designing a halfwave microstrip dipole(not a patch antenna), which consists of two rectangular monopole strips of quarter wavelength each. I am simulating it in HFSS and I cannot achieve the 2.14dB which is the theoretical gain of a single dipole. My gain is around -2.5dB.
Can anyone suggest the possible reasons for the reduction in gain?

thank you

The distance to the radiation wall is too small.
Also check the port...and the dimensions you are using

Thanks for the reply.
The radiating air box extends lambda/2 on each side of the antenna. I believe that it is a large enough airbox to get accurate results.

Is your antenna tuned?....Check the impedance
and adjust the lenght to tune the antenna...