Rcov/SimpleCov will only report the coverage of the tests ran.
For a full coverage report, you must ensure that the full test suite is ran as the last test, to build a full coverage report.
You will also want to make sure it is the first require in your test_helper.rb file.
From the documentation:
Note: If SimpleCov starts after your application code is already loaded (via require), it won't be able to track your files and their coverage! The SimpleCov.start must be issued before any of your application code is required!