What method did you use?
Retrobrighting is always a variable process as every company makes its products with different plastic compositions, surfaces and tonalities, and the products we all use for the treatment also vary a lot. If you removed the yellowing from the key and those stains remain, if the tone is the same right now as all the other keys you treated, there's a risk that doing it again might turn it lighter in tone in comparison with all others, and with no guarantees that the stains you described will disappear. It's part of the risk of this process. I wouldn't risk it any more than you did already, keyboards are very risky to retrobright, especially keys.
An example among others, I destroyed a Logitech Access Keyboard by attempting to retrobright its very yellowed keys. One simple submersion session and they came out in a sickly white tone with the text highly faded, and proceeded to break as I re-inserted them from being so brittle.
Having retrobrighted so many different things, I can confirm that between 6 months or a year and a half they all went back to yellowing, although in lighter tones, but it's just not worth it. And most of them were kept in cool and totally dark places, there's just no use, it will happen eventually.
If you really want to do more treatments to other things I would recommend this new method shown here by RMC Cave at minute 23 which is contactless with the plastic: https://www.youtube.com/watch?v=InoGKNFwPJs
My advice: just clean your stuff the best you can, keep it from direct sunlight and enjoy it for how it is.