Perhaps your tasks are not yet tab-indented under their Project headings ?
All parent-child relationships (project-task, or task to subTask relationships) in TaskPaper are consistently expressed by tab-indentation.
That can mean that a list of tasks which you intended as children/members of a preceding project, are actually just following peers or siblings of it, rather than belonging to or descending from it.
That might be why, when you click on a project in the sidebar, you get the impression that it is ‘collapsed’. (In fact, for lack of tab-indentation, it just doesn’t have any descendants yet.)
To fix it:
Go back into a full view (click the filename at the top of the sidebar)
Select a series of tasks that you want to belong to an immediately preceding project line
hit tab to make them indented under (i.e. actual members of) that project
To put it another way – TaskPaper files are tab-indented plain text outlines.
The project colon allows for special formatting, sidebar display, and line-type searches, but projects are polite, they don’t automatically assume that they are entitled to ownership of anything that follows them.
One advantage of the tab-indented plain text outline approach is that after or between projects, you can still make simple lists of unattributed items which don’t belong to any particular project, and which won’t be greedily claimed or snapped up by the last project which happens to precede them : - )