I am using a combination of many AD5941 chipsets and I run into the same problem once in a while where I cant successfully wake with AD5940_WakeUp(10). The boards will be working great but the random one will get into an endless loop where it cant wake and the ADIID Register (x400) returns 0xA0A0 eternally instead of the expected 0x4144. This is fixed by a power reset.
This is using a variant of the amperometric example.
What are the expected causes of this? I suspect that it could be due to brown out but I read that I should be getting 0x8080 when this happens.