Please know, some of the information you are asking here is considered in-character information, and should be discovered in game. Also, know that scouts have not been a forte of mine, so I may not answer these questions exactly correct.
Mifador wrote:
1) I seem to miss a lot of ambushes, meaning my first arrow hits but doesn't do extra damage. Is this because the skill is not mastered ? I thought better at ambush meant extra damage not better chance to ambush someone. If that's the case does mastered ambush give you 100% chance to ambush?
The exact increase to how extra much damage (if any) that occurs as a character masters ambush is information that is not usually given out. Please see the next response for more information pertaining your other question.
Mifador wrote:
2)In case it's the camo failing is it possible to have mastered camo and still fail it ?
Yes. "Mastered" in Shattered Kingdoms does not mean you will ever be perfect at the skill or spell. Instead, "mastered" is used in place of "perfect" to imply that you have attained the highest level of perfection possible - not that you will be perfect.
Mifador wrote:
3) Does loading your bow with an arrow break your camo ? If that's the case should I reload first and then camo ?
Please feel free to ask this question in game.
Mifador wrote:
And 4) I failed to get a skill increase by spamming camouflage, should i get visible first or am I just unlucky ? Can you be camouflaged and break it by trying to camo again ? There isn't any message to tell me if the skill succeded (like sneak)
Again, please feel free to ask this question in game.