Soldiers were not as trained as much. Soldiers were not land owners, but maybe could be later... (Harder to fight for something if you do not own it) Soldiers started getting paid on the outskirts of the frontier, and not from Italy itself. Soldiers were to over confident, particular the Battle of Teutorburg forest. Some barbarians had great leaders in battle. Some Roman generals did not want to be out in the field, they wanted to be politicians.