If a delete file/copy file operation doesn't work what seems randomly, and you've ruled out that the paths are correct what else may cause the problem?
If the files are scattered throughout the folder, you may be able to make them contiguous by grouping them. For instance, if you want to grab all of your .png files, right-click a blank spot in the folder window and select Group by>Type. If you want all of the files altered last week, select Group by>Date (date last modified). You can select Group by>More for additional options.