I have a lot of media and when I try to drop them, ingress says "drop successful" but items still in my inventory. By the way, I can't drop other stuff too.
You can't drop items if GPS position is not correctly known (when your scanner is scrambled).
If your position is correct and app says "drop successfull" try a "force sync" (it is in the ops=>device menu) to check server state.
If it still doesn't work, it's a bug.
You also might try clearing the cache and app data to make sure that your inventory isn't 'clogged' by an invalid item. I had this problem once, it kept saying that the item couldn't be dropped because it didn't exist.
I force synced a few times, then eventually forced the app closed, and cleared both the cache and stored data. This forced the app to reinitialize and gather all user info from the server instead of a normal cache update.