Ok, i understood what happened.
Fact first : extending a volume (usually) requires a call to grow partition first (it is worth reminding us that a partition and a volume are 2 different things).
One could say that I could/should split these 2 tasks in CloneDisk (rather than hiding these 2 operations in one action) but anyway, not the point here.
If you clone a volume (sitting on a 250gb disk) to another volume (sitting on a 500gb disk), you actually need to have a partition existing already on target disk (normally as big or bigger than the source).
Therefore, when cloning a volume to a volume, the partition length may become inconsistent with the volume length.
You can actually see this in partition editor in clonedisk.
In your case, your partition length on target disk is probably around 500gb (and probably at its max length).
That will prevent the call to grow partition to take place and currently stop the whole extend operation in CloneDisk.
No big deal, in your case, what you "only" want to extend volume (the partition is not your concerned here).
Latest Clonedisk version now handles this particular scenario : it will tell you that grow partition failed but will ask if you want to continue anyway with the extend volume part.
With that latest Clonedisk version, i was able to work around the issue your reported.