Your question seems to be more related to image brightness when displayed rather than "exposure." This comes down to having a calibrated monitor and screen brightness appropriate for the ambient levels.
A tool I use is a gamma test strip... I have it embedded into the Lightroom interface as the "identity plate."
If I can't see the difference between the ...
The most common issue is faulty contact/communication. Make sure the pins/contacts are clean and the flash is fully seated. There's a little bit of play in the fit of the foot in the hotshoe, you might want to try wiggling it to either side.
Though this is little more than my own opinion, I think it's lacking in punch.
Even for a low-key image, I'd usually push the whites to white - lift the specular highlights.
If you were looking at this scene 'live' the store lighting would be really quite blinding compared to ths darkness your eyes were acclimatised to. It would be pushing that light out ...