This could be many different. Check the following as these are usually the most common.
- On the Edit Quiz page (e.g. where the questions included on the quiz are shown), check to see what you have entered in the Maximum Grade field.
- On the same page, check to see what you have each question marked out of. If they are zeroes, that means that none of the questions have any weight.
- In the Gradebook, check to see if you have the quiz item locked. This would be on the Categories & Items page. If it is locked, grades won't be updated until it is unlocked.
- In the Gradebook > Grader Report, do you have the grades overridden? They would be indicated by an orange tint to the cell.
- If none of the above, have you tried running a regrade?