As the title says, Upping the reinforcement to max is usually granting a ranged capacity to those units, but I can't see them shooting at anything, just never, grounded or flying targets, without any other battle engaged, it's the same.

What am I missing ?

(I'm used to the first Kingdom Rush, and I didn't have this issue with the maxed reinforcement)

Is that on Heroic/Iron challenge?

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