Select the textbox and select Custom Automation in the Slide Show menu
item, then Add Effect/Entrance, then in the Modify Effect pane select
the Text Box's item, pull down the dialog and select Effect Options
and check the Text Animation tab. Its Group text: item should read
'by 1st level paragraphs' for the major bullets or lower levels, if
desired.
Generally, this behavior is the default when a text box has a custom
automation set for it, but it doesn't hurt to check.