Make sure you've spent as many skills points as you earned Job Levels-1 on your first class. You can't spend points in your second class skills until you have used first class job levels up. For example, if you level a mage to JLvl 50 you have 49 points to assign. If you change to wizard and level and then reset your skills you still have to spend 49 points on mage skills before you can put any in Wizard skills.
