At level 1, my dark elf has Conjuration 5. I leveled it to 15, and then 18 and on both attempts to level up it only gives me +3. To contrast, I leveled H2H (7 levels) and Blade (3 levels) to get the full +5 for Strength, as well as Destruction (8 levels) and Restoration (2 levels) to get the full bonus as well.

Just tried another combination of Conjuration 9 and Mysticism 1, still +3. =/ Is this some disease or a bug? Playing on the PS3 btw.

In order to get a +5, you have to raise skills related to that attribute 10 times before you get the "Level Up" message. Is it possible that some other skill-up (of a Major skill) triggered the level-up before you got the 10 skill-ups?