The most common reason why the auto top-up is not triggered is because the balance never exceeds your specified minimum balance, as the balance is always below.
To avoid this happening, set the rule so that it is topped up with more money than the specified minimum balance. See example below,
Other possible reasons:
AutoTopup has already been triggered
There is already an invoice open towards the customer that has not yet been settled
One thing to keep in mind is that we’re triggering on actual account balance (not taking into account non-settled transactions). This causes an additional “delay”. For example if the purchase that seem to traverse the threshold takes 2 days to settle, then the auto-top up will trigger 2 days after what the customer might expect.