Do you have the Invitation that is available right now or a different one? If you have an earlier invitation, you have to be on the specific Planet the Strike for the Invitation is on. If you have the new invitation, I'd try to track your invitation and do Multi Kills if possible, that worked the best for me. You can also try to clear your console cache or run the Scan and Repair option depending on your Platform prior to trying this if it's a game issue.
Please double check your pursuit to make sure you're attempting to collect samples on the proper destination. Read the description.