When cleaning with the toothbrush, did you remove the key caps or just brush around the caps? You might have got something stuck in your switches, which causes them to not actuate properly. I would recommend taking the key caps off (if you can) and use some compress air to blow out the switches to see if you can remove some of the dust / dirt particles that are probably causing the issues.
I would repeat and instead of a toothbrush, I would utilize some low compressed air spray. With these being optical switches, they are more acceptable to dirt and dust particles so brushing with the toothbrush may have caused some of that to get into the switches. You can focus on the keys in question to see if that makes a difference.
Using a toothbrush in general won't have hurt the keyboard, so most likely just got some thing in those switches that the air should be able to move for you.
Take the keycaps off those keys again and push the switch under it and see if it some issue with how the keycap fits or you somehow damage the switch itself.