Some skill become unavailable upon equipping new gear

My Orb of storm disappeared from my skill bind when I equipped a new armor. What did I do wrong? I checked my character stats, all seems fine. So.. what happened? I tried to re-equip the old armor, but wouldn’t let me due to not meeting Dex.

I assume that you have attributes on your armor and then either the requirements for the skill or one of its support gems is no longer fulfilled. You could just remove the skill gem from the menu and then try to put it and the support gems back in one by one. It should show you why you can't.

Edit:
Keep in mind that you can respec some attribute points on the passive tree to maybe be able to fulfill all requirements.