Items Disappearing

Things are disappearing from people inventories on my Realm. My friend logged on and his elytra was missing. Another friend logged on an his elytra and fishing rod were missing. I had a nether star in my inventory and like 1 minute later I noticed it gone. I have no idea what to do and am very disappointed as I not only payed for this game but am also paying a monthly subscription. Plz help ASAP