On a natural nail overlay you must be very vigilant at spotting potential problems not just ones that are highly visible.
The most obvious potential problem area with NNO's is at the tip where there can be separation or 'curling', cracks or signs of stress ... otherwise the rebalance procedure is not really any different to a normal rebalance.
You need to start at zone 1 as usual with your file prep. Shorten and thin out in that area taking note of any problems you may see so that you can address them on application.
Then move on to zone 2 and be careful to check the side walls for any signs of stress fractures (if you see any then you must thin out and remove the cracks).
Finally zone 3 where you need to check for lifting and remove it if there is any present.
I highlighted 'start at zone 1' because many do not do this but head straight for the obvious problems they can see. Take it systematically zone by zone and deal with any problems along the way (it is easier this way).
If you are doing a French rebalance on a NNO then my advice is to use a sculpting form under the free edge after you have done your preparation ... to me this produces a finer result, gives you more area to work on and also nicely caps the free edge and sides of the free edge to give more protection.
I hope you get on better this time, Raya.