When you do the repo sync, redirect stdout and stderr and then grep for errors to make sure you got the full source code:
repo sync [other options] >& sync.log
Likewise when you do the build:
make [other options] >& build.log
Examine the build log for errors. Don't share the full logs
by either attaching them or including inline,
they are usually far too long. Just include the relevant
error lines and enough context to be able to understand.