Is that on Heroic/Iron challenge?

Yes mainly, I don't remember seeing this issue on normal levels.

But I'm today playing right in a normal level and they are finally attacking with their spears. Maybe my game needed a restart and a long night sleep :p

edit : ok I just figured out that reinforcement are like lvl 1 in heroic/iron, no matter what level they are upgraded. Is it same for any category of upgrades ?

Yes, Heroic/Iron challenges have upgrade restrictions that applies to all category.

You could see it in the stage description - how much level of upgrade is allowed for that particular stage.

So reset and re-add your stars to those that is below the restricted level then just re-assign them when going back to main stages.

On later stages the restriction would be upgrade level 5, so there'll be no restriction at that point.

Haaa it makes perfectly sens since it's the exact same icon. I just totally missed it and was thinking it was only for the towers during the game.

I'm so tired of myself sometimes... T_T