Saturday’s 4-1 defeat at leaders Chorley hurt the former Nantwich boss.
Ahead at half-time, had they been more decisive in from of goal he feels they could have been out of sight at the break.
Parkinson said he didn’t “roar” at his players but told them their second half collapse can’t be repeated.
Alty host in-form Southport this weekend - a game which could prove to be crucial with both sides now gunning for the top seven.
“We could have been 3-0 up at half-time at Chorley but we weren’t,” he said.
“We spoke at half-time about how we weren’t clinical enough and how that allowed them to come back at us and if did.
“I was disappointed with the application. When you are a goal ahead at the league leaders you need to show character.
“Chorley came at us all guns blazing. The game will showed me where we are.
“My teams always fight until the bitter end and we didn’t and that’s a real frustration,” Parkinson added.
“I believe in the group we’ve got and I’ll stick by them but they have got to start learning quickly.”