I'm guessing the reason you're not able to assign skill points below the first row (or advance to the next tier) at Level 14 is that you are distributing your skill points to two or all three skill trees. I recommend focusing your skill points to one skill tree only, as 5 points assigned in a single skill tree will unlock the next tier (row) of skills on that same skill tree only. This makes it preferable that you stick to one skill tree only, if you want to advance to the next tier.
From the Borderlands wiki 'Skill' article:
Borderlands 2 features three skill trees per character.
A player chooses skills by investing skill points, awarded as a
character increases in level. Each time that the character increases
in level, they receive an additional skill point. The first skill
point is gained at level 5, and must be used to acquire the Action
Skill for that character's class. Subsequent skill points may be
invested in a number of different skills that unlock as the character
Most skills have five levels, each requiring one skill point to
unlock. However, the Action Skill and some other skills only have a
Each character has three skill trees and each skill tree has six
tiers. The first tier is the top row of the skill tree and is
available for spending Skill Points on as soon as the Action Skill of
the character is unlocked. Every five skill points spent in a tree
unlocks the next level of that skill tree.
Tiers are unlocked by the total number of points invested in each
It doesn't matter where in that skill tree you assigned your points. 5 points anywhere in a single skill tree, will always unlock the next tier in that single skill tree.
You should experiment with the Borderlands 2 official website's Skill tree builder to find out how exactly this works.
For a certain fee, you could always respec your character (reset the assigned skill points, so you could reassign it), and get to the next tier of your preferred skill tree.