If you are using the LTI 1.1 Canvas integration and a submission does not appear in the Canvas SpeedGrader but is available within the LTI assignment inbox, it may indicate that the student has submitted directly through the Turnitin website rather than through Canvas. Please double-check how the submission was made with your student.
Submissions made directly to the Turnitin website will not be passed onto SpeedGrader. Therefore, students will need to submit again through Canvas to have the submission show in SpeedGrader.
Please note that if you are using the LTI 1.3 integration with Canvas, submissions will not show in SpeedGrader. You will need to access submissions and their Similarity Reports via the LTI assignment inbox. This is expected behaviour.