Colcon test fails with "collection failure"

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When I try and run the 'colcon test' command it fails with:

Starting >>> my_package
Failed   <<< my_package [0.93s, exited with code 2]

Summary: 0 packages finished [1.31s]  
1 package failed: my_package

The error message displayed with 'colcon test-result --verbose':

build/my_package/pytest.xml: 1 test, 1 error, 0 failures, 0 skipped
- my_package
      <<< error message
          collection failure
      >>>

Summary: 1 test, 1 error, 0 failures, 0 skipped

Does anyone have any idea what's going wrong here in this scenario? I have checked the pytest.xml file and it's identical to anoteher package file I have in another package which has no problems with the colcon test.