Stuck on Workflow

Hi,

As soon as "Test Sent Date" is entered & "Test" picklist field's value is NULL or NR, then "Test Received Date" should be updated & a notification should be triggered. I've applied below workflow which seems to be working only when Test = NULL but not when Test = 'NR'. Can you please correct what I'm missing here.

IfNull(PRE('<dTest_Sent_Date_1_ITAG>'),'NOT GIVEN')<>[<dTest_Sent_Date_1_ITAG>] AND [<dTest_Sent_Date_1_ITAG>] IS NOT NULL AND IfNull([<plTest_ITAG>],'NR')<>[<plTest_ITAG>] AND [<plTest_ITAG>]='NR' OR [<plTest_ITAG>] IS NULL

Appreciate quick help please.