AD8237 cannot amplify the signal at micro volt (uV) level.

I use AD8237 as the first stage to amplify the EEG signal (5 uV to 100 uV). The gain is 10. The second stage is OPA2333, gain is 100. Power supply is a button battery. But I have a serious problem now, the signal at the level of uV cannot be amplified accurately. Here is the schematic:

If the input is a square wave,

The output is

The gain of max and amplitude is about 10, correctly. But the gain of mini is incorrect.

I use a PicoScope 2200 USB oscilloscope, it cannot show a uV signal. So when I use a voltage divider to generate a 100 uV signal, I test the final output, but there is nothing, just noise. I think that the first stage AD8237 does not work. 

Can anyone give me some advices?  Maybe the problem of reference pin? Offset voltage?......

Thanks very much!