There is now a glitch that occurs in training mode where the item may fail to increase the weight of the user until it is attacked at least once. (ssbwiki.com)
The new Training mode acts as an optional tutorial that explains the fundamentals of the game as well as detailing some of the newly added mechanics. (thegamersparadise.com)
There is a handy training mode for this purpose. (thevitalounge.net)