you can use player.showinventory to show all items in your players inventory, i had the same problem and there were just a bunch of hidden items weighing me down. actually, in a heavy modded game, not always every "hidden item" will pop up in the "showinventory" command. So if that doesn't work alone, go to any crafting station and than use the same command, you'll be shown some things that were invisible and didn't show up while you tried to check for them (eg. go to a forge and input "showinventory" and you'll see most of the ingriedients for forging, go to a potion station, do the same and you'll find some ingriedients etc.)