Hi there,
On my poly 61 i cleaned the printstroke under the keyboard by screwing it off, removed the rubbers ang cleaned it with a soft cloth and ALCOHOL (yummy !)

Another posibility is to try somthing with a pencil,i think you need to put some pencil dust, (not the wood) on the black spots on the same print stroke as mentioned above.

I hope it works for you, i had enough on cleaning... perhaps the alcohol made me think it worked better..?