Editing custom Xenial iso by Isomaster make it unbootable

[danjde]

Hi friends,
I'm building my custom Ubuntu live iso, from Ubuntu Xenial Gnome.
For build I do help by JLivecd and Cubic, and each work really well.
And now, my question is:

using Isomaster for edit the iso, I obtain a non-bootable iso image.
The last year, building my custom Linux live from Ubuntu Trusty Gnome, I've not encountered any trouble, I was able to edit again and again the same iso, without problems.
Why now, seem impossible to use Isomaster for edit the new Xenial iso without lose the boot ability?
Is it different the Xenial iso structure?

many many thanks!