You installed BI components for AX 2012.  Prior to this you had made sure the Analysis Servers form has the SSAS\NamedInstance marked as the Default and Report Servers form in AX has the correct Configuration ID marked as the Default.  

You ran the SQL Server Analysis Services project wizard and followed the steps in the wizard.  You selected your SSAS\NamedInstance in this wizard. So, now you have your OLAP Dynamics AX database on a SSAS named instance.

You have also published your AX reports to the SSRS instance.

Now in SSRS Report Manager page, if you look at the connection string for the DynamicsAXOLAP data source the Connection string here is still referenceing the default Instance of SSAS!

So this is something you need to be aware of.  When you publish SSRS report running the command "Publish-AXReport -Id SSRSNamedInstanceConfigID -ReportName*" and even though your Analysis Servers form has the SSAS\NamedInstance marked as the default, our installer is not aware of your SSAS instance.

Here are the additional steps/workaround you will need to get the Connection string in the DynamicsAXOLAP data source -

1.  Manually edit the connection string in DynamicsAXOLAP data source.

2.  Open the Dynamics AX 2012 Management Shell and run the following:

Get-AXReportDataSource -DataSourceName DynamicsAXOLAP| format-list

The results indicate the datasource that will be used during the reports deployment.  Notice "Data Source=localhost".  So this means default instance of SSAS.

Since your SSAS is a named instance you will need to change this so the DynamicsAXOlap Data source gets created with the correct connection string.  You will need to run the following command:

Set-AXReportDataSource -ConnectionString "Provider=MSOLAP.4;Integrated Security=SSPI;Persist Security Info=True;Data Source=SSASServerNme\SSASNamedInstance;Initial Catalog=Dynamics AX" -DataSourceName DynamicsAXOLAP

You will need to delete all of the AX reports/components out of the report server and redeployed all of them again.  Verified the DynamicsAXOLAP datasource is updated to the new instance of OLAP.