Hi,
one possibility is that the forward/reverse signalling is not symmetric. If the slave were losing or gaining steps you would expect it to happen in bunches,
not just slowly accumulating over time.
Typically a slave motor turns in the opposite direction to the master motor. Thus if the Master is in FWD the slave will be in REV.
If the stepper drivers have say a pull up resistor on the Step signal it might be that the master stepper motor goes 1000 steps FWD and then 1000 steps REV
to end up in the same spot. The slave axis may go 1000 steps REV but then only 999 steps FWD. Over a period of time the slave will lag behind.
To test this you need a long stroke dial indicator, and even then you must be very particular to get the accuracy required to tell whether this is happening or not.
Craig