Hi Zercon,
I am confused (not an uncommon thing), and you are irritated. So, you can either swear at me or help to alleviate my confusion a bit. EIther one, I don't mind... although I would prefer to lose the confusion either way.
So, you have hit level 5 (Character level, I assume), and it is showing that you have the expected level 5 Armor XP cap of 9 points, but that you currently have 0 Armor XP points.
So you're not earning Armor XP, and probably also not earning the Tactical/Sabotage/Assault-specific Armor XP points either, correct?
As you are level 18, you will need to be fighting, and being hit by, mobs that are at least level 15 in order to get XP, although I would recommend level 18 to 20 for leveling.
First question: Which armor type are you using? Tactical, Commando/Sabotage, or Assault? Or a mix of the 3?
Second question: How many armor pieces do you have equipped? All 7, or less?
Third question: When you get into a fight with one single mob of the type you are usually fighting, roughly how much does your Shield number go down by every time you are hit?
If the answers to the above questions are "Assault", "7", and "some number less than 20", then you are below the threshold for earning Armor XP with each hit you take. If the mobs you are fighting are around your level (17-19, say) then that probably means that even as a t3 set the armor value is too high, making me think you are using Assault armor, which is definitely the hardest/slowest to level. You can try to unequip the armor piece by piece, and you will probably find that at some point you start earning Armor XP.
Normally, I would suggest leveling with Tactical armor, as it seems to give the highest Shield value of the 3 (especially when wearing a full set) and it blocks the least damage, meaning more Armor XP per fight. You can then use the Armor XP you get with a Tactical set to unlock your Assault levels, swapping to Assault when you get past the level-grinding stage. However, if you are going to stick with Assault, try stripping armor until you see the Armor XP indicator moving up during the fight. I am pretty sure it will work, but let me know either way.