In Appearance >Widgets panel you can see the list of available widgets and widget areas. Here’s how they work - each theme has default widget areas, such as Footer widgets, Header Left widget, Header Right widgets, etc.
Some themes have custom widget areas made for each homepage in order to have different widgets on them. Custom widget areas are added in Appearance >Widget Areas panel, by selecting which default area they will replace and on what page(s) they’ll display.
Once you choose homepage, go to Appearance >Widgets and check if that homepage has its own custom widget area. If it does, you have two options – first is to move widgets from this custom widget area to a corresponding default area. Other solution would be to simply assign custom widget area to display on all pages by checking them in Appearance >Widget Areas panel.
*Example is from Celeste theme.