I know this is an old thread so I don't know if anyone else is still experiencing this, but I have a household who has completed the Feathers and Postcard collections but haven't received any plaques for them. I've checked their household inventory and they're not in there.
Like others have posted above it does seem a bit hit and miss as to whether you get the plaque or not. In a different save I have, the household completed the Snowglobe and City Posters collections and did get the plaques for those, but in another save the household finished the Message in a Bottle collection and didn't get the plaque for that.
One way to get those plaques is to move. Place all the collectibles in a sim's inventory, then split from the household and move that sim to a new lot. Wait a day or two until new plaques show up in the mail. Then move the sim back to their original household. Then a few days later, you should get them again. Then delete all the extras.
Hope this helps.
