Why was the Swedish visa refused?

Why was the Swedish visa refused? The reason for refusing a visa is sometimes the material problem of the visa applicant. The materials are incomplete or cannot reflect their own advantages, and those with immigration tendency cannot pass. At this time, professional organizations are needed to help. My sister-in-law also looked for an intermediary before, but she was refused a visa several times. Finally, she found the reaffirmation network and passed it easily. It's state-run and has a hard relationship at the embassy. Without this, it naturally costs money.