If you decide to look at things from a different perspective, and take into account only the people who finished the onboarding, you will find out that 20% of them came back to your app.
600 users vs. 300 is not so bad after all.
Now… which report is relevant? How will you interpret the data?
If you take into account the first results, this means you have a retention problem and you should start a campaign to increase your first week retention.
But the second report points out that retention is not really the issue here. The real problem is getting people to finish the on-boarding process, which would require an on-boarding optimization campaign, not a retention campaign.