Based on the raw data and the combat code, it is possible that the amount being given by the AA line is too small to parse.
The following data may be helpful in testing the ability again:
The description that states that you must be dual wielding piercing weapons is incorrect. The AA increases your chance to parry regardless of your weapon setup.
Here are the raw values for each rank of the AA:
Rank 1: 2
Rank 2: 4
Rank 3: 6
The Parry spell effect is cumulative as an AA effect but only the one highest buff and one highest worn item is added to your total.
Improved Parry / Block VII: 56
Parry does not apply to incoming attacks that are throwing or archery.
Parry won't work if your back is to your attacker.
Active defensive skill order (succeeding an earlier skill will mean the later ones won't bother checking):
Then regular strike-through is checked.
Then your chance to simply avoid the attack is checked (defender's avoidance roll beat the attacker's accuracy roll.)
Your chance to perform a parry is:
Integer math so truncate every division.
Chance = (((SKILL + 100) + [((SKILL+100) * SPA(175).Base1) / 100]) / 45) + [(hDex / 25) - min([hDex / 25], hStrikethrough)].
If random(100) <= Chance: You parry
(If chance >= 100 then 100% success rate for parry.)
hStrikethrough values - if testing using a guildhall combat dummy at their default levels:
Meda and earlier: 0
Assuming you're a level 105 Rogue, have an item at with Imp Parry/Block: VII, have no parry buff, are fighting an NPC with no hStrikethrough and have 800 hDex:
Without AA: Approximately 51% chance to parry.
With AA: Approximately 52% chance to parry.
Not a substantial increase certainly, but it should technically be there.