Greetings! Sorry for inconvenience, it's a display bug which causes the loot indication board doesn't show up at times. Actually, the loots will be directly added into your inventory bag, sometimes with a delay, maybe several hours according to situation.
If you are still not sure about that, you can try to clear the notification of the items in your bag before a titan close to defeating, and check your bag again after its defeating, you will see the loots are already there. We're sorry to bring you this confusion, this issue will be fixed in future update, thanks for your understanding!