First of all, it's not "not giving an offset". It definitely has an offset voltage but since your simulations are run with perfectly matched devices you're just not seeing it. Run monte carlo and then start stressing about the offset. Also even if your devices are perfectly matched you'd still see a very small offset which is the effect of finite gain of the amplifier.
My best guess is that since this is a fully differential amp, you have some kind of asymmetry in your layout causing this. It is interesting as I wouldn't think it would affect the DC performance this much, but I'm not sure. But anyway 20mV offset without any imperfections is still too much. So before going into augmenting it first make sure this is working at its best.
For augmentations, there are lots of things you can do, simplest is just trimming it. But you need to be able to measure it and correct it, which is not a very easy thing. Also you can try chopper kind of stuff. There are devices with offsets as low as uVs using this kind of architecture.