At Most One Record Can Be Returned By Subquery After Appending New Rows

I have a subquery on a table which got the previous and next rows from a table. This worked fine, until I started replacing my table with real data instead of dummy-data. I now receive the "at most one record can be returned by subquery" error every time I run it. I checked that I do not have any identical rows.

My query:

Code:
SELECT z.DateFrom, z.DateTo, z.ArticleID, z.retailerID, z.Actuals, z.Statistical, (SELECT y.Actuals FROM tblActuals AS y WHERE (y.retailerID=z.retailerID) AND (y.ArticleID=z.ArticleID) AND ((DatePart("ww",z.DateFrom,2,3)-1)=DatePart("ww",y.DateFrom,2,3))) AS ActualsMin1 FROM tblActuals AS z;

I append new rows via a VBA-script from another table.