The signal may be out of the acceptable common mode range. Please refer to the table from the datasheet (Rev 0; 3/15) below. When the MAX11214 is in PGA mode, the input voltage needs to be larger than VAVSS+0.3V. Please use a dual power supply when the input is millivolt-level input signal with small offset.
The figure below from the datasheet provides a visualization of the table above.