The Blues have scored more than one goal in a match just once since the former Crawley Town boss took charge in December.
Barker has admitted that while the team's defence has improved under his stewardship, he still has work to do at the other end of the pitch.
"We had an issue with conceding goals and – although we haven't solved that completely – we're harder to beat now. Now we need to move on from 'harder to beat' and start winning more football matches," he told the club's official website.
"We'll keep working on it and keep trying to do the right things on the training ground.
"If we have a bit more confidence about not conceding, then we can be more positive in our play, which will lead to goals."
Portsmouth currently sit 19th in League Two.