Oh yeah, the inconsistency is driving him absolutely MENTAL!
Anyway, I gave it another go this morning. Doused another cotton tip with rubbing alcohol and rubbed away - nothing. Then I took a make-up removal pad, put some rubbing alcohol on it, then put my finger nail behind it to scratch at the smudge. I had to give it a really good solid scratch, but the sucker FINALLY came off! Guh... that was one helluva stubborn smudge!