Ben Close admitted Pompey must stamp out their defensive lapses if they are to find consistency in League One.
The Blues suffered a third successive defeat against Bradford on Saturday, despite dominating the game for the majority.
Matthew Kilgallon was left in acres of space inside the box on 80 minutes and nodded Tony McMahon’s free-kick home beyond a helpless Luke McGee.
It was a disappointing goal to concede on Pompey’s behalf and Close insisted the errors must be eradicated.
The midfielder said:
‘We’ve been found out a few times now.
‘We are making little errors and they are turning out costly.
‘We have to cut that out and we did do that on Saturday.
‘It was a different error than in defeats against Doncaster and Blackburn.
‘It was a set piece and sometimes they are hard to deal with, especially when they have quality like they did.
‘We need to learn from those and not be conceding from set pieces to give us the best chance of winning games.’
The Valley Parade outfit made Pompey pay for the high number of chances they missed.
Close, who had an effort saved by Bantams goalkeeper Colin Doyle in the second half, revealed the Blues must also become more streetwise in the final third.
He added: ‘Sticking the ball in the back of the net is the hardest part of the game.
‘We have got players who can put the ball away.
‘Saturday just wasn’t our day. We couldn’t find the goal and ultimately we have been punished for it.
‘It’s a tough blow to take but we have to focus on the good points of the game.’
– WILL ROONEY