Is the table corrupted??

[RedHatter]

I have written a query which pulls data from a table tasks and a table class.
I want the result of the query to show the list of tasks completed by the studnet and also the tasks not completed. I used Is Null to perform this but notr all of the records from the task table are displayed. Some are displayed but other only when the task is completed.

I did delete some of the records in the task table and am not to sure if this messed it all up , resulting in the above.

I changed the join type but this made no difference

PLease see SQL below.
SELECT [Stocker (Dos) Tasks].StockerTaskName, [Stocker (Dos) Class].[Stocker(Dos)Tasks], [Stocker (Dos) Class].PrdPerformed, DateAdd("m",[Ranking],[PrdPerformed]) AS Recert, [Stocker (Dos) Class].StudentName, [Stocker (Dos) Class].PrdRead
FROM [Stocker (Dos) Class] RIGHT JOIN [Stocker (Dos) Tasks] ON [Stocker (Dos) Class].[Stocker(Dos)Tasks] = [Stocker (Dos) Tasks].StockerTaskName
WHERE ((([Stocker (Dos) Class].StudentName)=[Enter Student Name])) OR ((([Stocker (Dos) Class].[Stocker(Dos)Tasks]) Is Null));