Reset All Rows When One Row Changes

I'm trying to create a trigger that will clear the "checked" field when all items with the same classification have been checked.

The purpose of this is to do cycle counting of inventory, which is when you count the stock levels of a proportion of stock that have the same classification.

The checked field will be used to record the date it was lasted checked and will cleared once all stock have counted, putting back into the pool of stock to be counted.

Therefore, the trigger is suppose to count the number of rows that share the same classification as the amended row, and compare that with the number of rows in that classification, that have a date in the checked date field. When the two values are the same, reset all the checked dates to blank.

But when I tested it, nothing happened ...

ALTER TRIGGER [dbo].[UDEF_Last_Checked_Reset] ON [dbo].[ASC_PMA_TBL]
AFTER INSERT
AS
BEGIN
DECLARE @PartOnly varchar(16) -- Part Number
DECLARE @Rev varchar(4) -- Part Revision
DECLARE @PartCode as varchar(2) -- Part Classification
DECLARE @Last_Checked as datetime -- Checked Date

[Code]...