Hello,
Are you playing in a region other than your actual timezone? This problem is persistent for people that play in timezones other than their actual timezone. For example, if you play in GMT+0 but live in GMT+8, you'll always have issues with some events like this which have differing resets based upon whether they reset at midnight, 5am, server, or local time. For this reason, we recommend always playing in your actual region. As a workaround, you can try setting your computer's local time to match your server time when you need to claim fragments for an event like this, and it may work to properly set yourself to the "right" time for claiming. You c*so try putting in a ticket to Customer Support if you have screenshots showing you were online at the "right" time, and hopefully you can be reimbursed for your missing fragments.
Otherwise, if you're playing in a mismatched timezone you'll need to make sure you're awake to claim your fragments in the narrow window where you overlap.
Thanks!