please leave aside album sales cause we all know fans are responsible for those.
Mamamoo has always been a group relying more on public than fandom for their songs to succeed.
Hwasa and Wheein have had success with their solo songs pretty early in their career. Wheein and hwasa also have had way more features and collaborations with other (and famous) artists that put them on the public's map and paved a nice way for succesful solo careers, while solar has way less collabs and features than them for some reason and it took her extremely long for her to launch a proper solo career just for it to flop
But what held solar back? Her only serious solo song "spit it out" is a joke if we compare it to the huge amount of hype and success that any of Hwasa or wheeins solos have received (chart-performance and impact wise).
Ironically, solar who used to be the most popular member within the public with many opportunities and shows and whose visuals are actually most loved in the group has nevertheless been the least succesful member of mamamoos vocal line in her solo career. This is so baffling because mamamoo vocal line are so equal in skill, attitude and stage charisma.
I don't even see much anticipation or interest for any of Solar's solos as if the public has long forgotten she exists. Not even the fandom can make her songs chart.
I tend to watch mamamoo performances like starry night and egotistic, then gaze at her beauty and talent and wonder " how could she flop so hard? Why?"