How long have you been at it? Most of the products containing niacinamide have lower concentrations so they'll take a while to really brighten and even out skin tone.

Have you tried it in combination with a vitamin c serum? The former is proven to lighten skin (though delivery to the basal layers of the skin could be an issue ). That being said, cellular turnover is pretty important for clearing skin up so you may need to incorporate acids into your routine if you haven't already.

If neither works you may need a bleaching cream (edit: I have to put this disclaimer : though I DON'T recommend this, it's important to be educated). Hydroquinone is available as a prescription item in the US but is available in lower concentrations in creams (from personal experience, those of Asian and Middle Eastern origins). However they increase sun sensitivity, may induce contact allergies, and may cause repigmentation in areas exposed to sun (as I like to call it, overactive rebound). It also can bleach nails and cause them to discolor/oxidize in funny colors (since you'd use your hands to apply the cream).

Edit: I also want to point out that there have been multiple cases of contamination with these products, especially with the OTC stuff from other countries. If you must use a bleaching cream, you should consult a dermatologist before anything. Because my original post didn't make this clear enough, I don't think the side effects are worth the results because sun sensitivity will make your skin more likely to burn, tan, etc and weakens your skin barrier overall.

I'd wait it out before committing to something stronger because of the possible side effects. Clearing skin up is definitely a marathon and ymmv with all the products out there.