My EGO Self propelled lawn mower is spitting out a lot of the grass. It has just started to do this, the bag is clean, the under carriage of the mower is clean? Why would it be doing this, it's only a year old.
Assuming that the grass is still in the same condition, not too wet or too high, and you haven't changed the mower height, I would suspect the blade needs sharpened. Also make sure that there is no build up of grass in the passage way from the mower deck to the bag.
There is some bag mounting info here - if you want to make sure you are mounting it correctly. It attaches on both sides.