Trouble With Stored Procedure

I'm having trouble with this stored procedure, it gets down to the insert and then times out.

Can anyone see why the insert might be wrong?

I did check and at least one of the workshops is open and the @Status is showing it as Open on the client.ALTER PROCEDURE [dbo].[AddNewWorkshopAssigned]

@Workshop_ID int,
@Client_ID int,
@Wait_list bit,
@DateEntered datetime,
@EnteredBy nvarchar(50),

@Status nvarchar(10) OUTPUT

AS

BEGIN

SET @Status = (SELECT
CASE
WHEN (tblWorkshop.Workshop_Status) = 'Canceled' THEN 'Canceled'
WHEN (tblWorkshop.Workshop_Size - COALESCE(COUNT(tblWorkshopsAttended.client_ID),0)) >= 1 THEN 'Open'
ELSE 'Closed' END AS Current_Status
FROM tblWorkshop INNER JOIN
tblWorkshopsAttended ON tblWorkshop.Workshop_ID = tblWorkshopsAttended.workshop_id
WHERE tblWorkshopsAttended.Workshop_ID=@Workshop_ID AND tblWorkshopsAttended.Wait_list = 0
GROUP BY tblWorkshop.Workshop_ID,tblWorkshop.Workshop_Size,tblWorkshop.Workshop_Status)

--Change Workshop Status if NOT canceled.
If @Status <> 'Canceled'
UPDATE tblWorkshop SET
Workshop_Status=@Status
WHERE tblWorkshop.Workshop_ID=@Workshop_ID

IF @Status = 'Open'

INSERT INTO tblWorkshopsAttended (Workshop_ID,Client_ID,Wait_list,DateEntered,EnteredBy)

Values (@Workshop_ID,@Client_ID,@Wait_list,@DateEntered,@EnteredBy)

END