Argyle were once again guilty of some schoolboy deffending in the 3-0 defeat at the Den just days after conceding five at Sunderland.
"We're not defending well at all away from home and you get punished at this level," Williamson said after the game.
"We were naive at times and when you stand in the box you get caught out and we didn't create much at the other end either.
"We let ourselves and the fans down. It is hard to finger why we are conceding the goals that we are away from home but there is a lot to do and we'll have to start by winning the next game.
"We are down there fighting and we need a reaction from the players at home. We must win our home games."