I don't get why "Primadonna" popularity doesn't reflect to the album sales
Because it's a pop song - and many people are buying it who are new to her and unsure about further purchases. Many pop acts find it hard to shift albums until they have several crossover hits. Many pop acts NEVER shift albums (Bananarama and Kim Wilde had real problems their whole careers).
I think it will be fine a couple of singles down the line. Next single will push it back in top 20. Third one will cement longevity (as long as it's Lies, The State Of Dreaming or Starring Role).
She needs ALL radio to support her, too. Not just Radio One.