ADPD188BI register read out is not change

The custoemr use ADPD188BI design to detect  gas or not gas. The use it in normal mode (AC power supply in open air situation status ). They read ADPD188BI data register directly ,but there's no change under cover or uncover status. But if use FIFO register read data out there's read out the write data.  So is there any diffent in those to situation? Please advice.Thanks a lot.