I would look at the exif data, I think you will find the flash didn't fire on the f/4 sample. Flash on this kind of subject can be tricky. Also another variable with this kind of shot is what type of metering you are using and where it read. You can be off just a few inches one way or the other and the metering will not be correct. The picture has quit a few light to dark spots that could affect your shot. If you can create this in other circumstances it may be something else.