What are the restrictions on assigning skill points?

You need to put 5 skill points into a tier in order to reach the next tier. It doesn't matter if you put all 5 into one skill or mix and match or if you put the points into a lower tier. The skills are divided into 3 groups for each character, so long as you put 5 total into one of those three you will be able to reach the second set of skills (for that group). If you put another another 5 in, you'll be able to reach the third set and so on. It's very simple once you understand it but they also never really come out and tell you so I can understand your confusion.

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.

Unlocking skills

Skill points

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 progresses.

Skill levels

Most skills have five levels, each requiring one skill point to unlock. However, the Action Skill and some other skills only have a single level.

Skill tree

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 tree.

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.