the thing is that if the first ost doesn't do well the other ones will not do well
because ost popularity depend on how the drama do
and drama ratings usually decrease with the time and don't increase
+ the first ost are often see as the music who represent the drama so if they don't do well the rest of the other ost aren't going to do well neither
HP first ost did well but the latest one the 1st season didn't do as well for example
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Actually the ratings for HP 1st season rose consistently throughout its run, starting in single digits and climbing to the mid-teens. Obviously 2nd season comes with hype so may start with greater viewership, but can't predict if ratings will rise or tumble. 1st OST released from 1st season was Lonely Night which did not chart well at all, whereas Jeon Mido's OST, which is the 2nd best performing OST from 1st season, was released near the end of the drama's run (I think 2nd to last). Joy's and Jo Jungseok's OSTs were released 2nd and 3rd.
But yes, this season's OSTs are definitely not performing anywhere near as well - in May 2020, there were 5 HP OSTs occupying Melon Daily top 10 (I know we can't compare pre and post-reform but ULs for Jo Jungseok and Jeon Mido were >600,000 and had longevity in/near the top, and it's not like these OSTs would have had fandom streaming). Whereas with this season's releases, seem to have boosted Lee Mujin's older solo release Traffic Light more than anything lol.
That being said, can't rule out future OSTs doing well, which may boost the older ones - eg Joy's OST was released in March, didn't start climbing until Jo Jungseok's released, even then climbed very slowly and didn't reach top 10 until May when Jo Jungseok was reaching the top and drama was reaching peak popularity.