Tried also NiZn in the past, have similar problems. First I thought my charger is bad but with another charger the same prob. Cells extremely sensitive for over-discharging. Because of the higher voltage mabe the device is still working whilee single cells are already over-discharged.
No matter which charging current I select, the current decrease, long charging times or never-ending charging.
This looks good, for me it´s OK if the last 4 cells are stored. I don´t think I will take a look some days later at these values because I will not remember which cell it was 
I use many NiMHs and know also this prob that the contact of smaller cells is not so good, if I try to insert or eject the cell on another slot the contact could get lost and the values are gone even if I react fast.