I had a syntax error (but not lexical) in my
source code and clicking on the grey [+] square
for expanding a collapsed folder did not expand
the folder (but the [+] mark has changed to [-]).
(The syntax error was in the method below the
folder I wanted to expand.)

But I could expand the folder by double-clicking
on the [{...}] mark. Once I expanded the folder
this way, clicking on [-]/[+] started to work.