There are quite a few reasons why your AWS Marketplace costs could be missing, low or incorrect. However, one area to check would be the 'Other' category of your costs history report. When a Marketplace bill contains reoccurring charges we categorize them under "Other". This is because these lines do not have text that identify them as "AWS Marketplace". You can see this in the 'ItemDescription' column of the 'Other' drilldown:
If your Marketplace costs are missing altogether, it could be associated to timing. AWS will often add in Marketplace charges mid-month following the close of the previous month. In other words, the invoice from AWS may be issued in the first week of a new month but Marketplace charges may be added in closer to the 15th of the month.