Conditional For Insert Trigger

Sort of new to this, hoping for some help here.
I've got a table which you insert Scores and comments for Different
Issues. Each User may only input ONE score, but many comments per
issue.
I'm attempting to run a trigger which looks at the distinct users (in
the table) and then will either allow the comment AND score to be
Inserted if the User hasn't put in a score for that issue yet. OR if
the User already HAS input a Score, set the Score to NULL and allow the
rest of the record to be inserted.
My trigger works, but instead of letting the *New* User to put in the
score, it just nulls it.
Sorry if this is kind of long, but I hope I've explained it
sufficiently. I hope someone can help me out. Thanks in advance
This is my trigger :
CREATE TRIGGER trCheckUserResponses
ON dbo.tblIssues_Responses
FOR INSERT
AS
IF (SELECT UserID from Inserted) not in (select distinct UserID from
tblIssues_responses)
BEGIN
Update tblIssues_Responses
Set tblIssues_Responses.Score= Inserted.Score
From tblIssues_Responses AS A, Inserted AS B
Where A.Response_ID = B.Response_ID
END
ELSE
BEGIN
Update tblIssues_Responses
Set tblIssues_Responses.Score = NULL
From tblIssues_Responses AS A, Inserted AS B
Where A.Response_ID = B.Response_ID
ENDPlease post DDL and sample data.
ML
http://milambda.blogspot.com/