Eventually all spirits not escorted to Soul Society will become a hollow. Some spirits are chained to a particular item or place that they couldn't let go of after death becoming a Jibakurei. A normal spirit that couldn't pass on is a plus, they usually wait for a shinigami/soul reaper to escort them to soul society. Pluses and Jibakurei will eventually become hollows and are prey for hollows that come to the human world. You can read more at Bleach Wiki.