In SharePoint 2010, you create a site page which goes under the SitePages pages library (http://<siteName>/SitePages/). You edit the page and add a filter web part. The filter appears to be added successfully, but you cannot see the newly added filter on the page. You can go on adding any number of webparts. Everything will be added successfully, but doesn’t show up in the page.
You can check the same by adding the query string ?contents=1 at the page url. You will be able to see the list of webparts in the page where you can see your filter webparts too.
You will not get this issue if you add webparts in a webpart page.
Reason: The problem occurs because the WebPartManager.DisplayMode property is not getting set when the page is edited. The web parts expect to check WebPartManager.DisplayMode.AllowPageDesign, but for these pages it is set to BrowseDisplayMode even when editing.
Solution: To see the filter web part you’ll need to select “Edit Web Part” for any of the existing web parts you already have on the page. You can then see the newly added filter web parts you have added to the page and also configure them.