Dungeon Defenders - not picking items and mana?

Items that are left on the ground are sold for their mana cost and split between the number of players. The same goes for mana that is not picked up.

Source: http://dungeondefenders.wikia.com/wiki/Mana

Items left on the ground at the start of a wave get turned into mana and split between the players.

If you lose the level you don't get the mana that is on the ground, it's lost because you lost the round.

NO.

in the end of the level, yeah the items are sold automatically. But during the level, there is a item limit for the dropped items and when they reach at the limit; they just disappear. Try to pick all of the items that drop a round and sell them all at the end of the wave. That will bring you more mana.