The background of your images are white or pixely making it hard to see the symbols in the placement dialog.
The block file was saved and the thumbnail image has changed
You must change your AutoCAD 2D model space Uniform background color to 0,0,1
Make sure you Apply and Save out of the dialogs.
Save and close your block drawing
Try the command again in CADWorx
You can also stretch the placement dialog to a larger size and get a clearer image. Look in the lower right of the dialog for that control.
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