Closing Opportunity but Status and Status Reason fields do not seem to update

I have a plugin that runs on the creation of an Opportunity Close event. Now, I want to perform an action when I close the Opportunity as Won. The problem is that when I close the Opportunity, the Status and Status Reason field values do not update to 1 and 3 respectively and on my plugin log it shows them to remain as 0 and 1 respectively. What could be the problem please? Thanks!