Project compilation question

I made a NB 8.2 project from existing sources.  The sources are in two directories, a and b.

In the Projects pane, ...

Under the project is a folder and under it are packages a.foo, a.foo.bar, a.foo.baz, ..., b, b.qux, ...

When I right click the project and select Build, many modules in a can't resolve references from b.   For example, a.foo.bar.java says b.qux doesn't exist when trying to import b.qux.quux.

But when I right click a.foo and select Compile Package, it builds without error.

When I iterate on Build / Compile Package, eventually Build succeeds.

How do I correct the erroneous Build behavior?

Thanks.

Kevin