The Blues’ 1-1 draw with Accrington Stanley on Saturday meant they failed to keep a clean sheet for eight successive matches.
What’s more, Pompey have also not registered a shutout at Fratton Park since the 2-0 defeat of Scunthorpe on March 16.
Sean McCoville netted for Accrington 44 seconds into the second half, with Ben Close equalising in the 59th minute.
The stalemate meant the Blues finished fourth in the table and will face Sunderland in the play-off semi-final.
Jackett admitted there’s been ‘blurred lines’ between who holds certain responsibilities in his side.
And the boss insists that must be remedied if Pompey are to reach the Championship.
He said: ‘In terms of our game, I do think we’ve gone off our responsibilities a little bit.
‘The lines have been blurred between who’s doing what and we have to get back to that.
‘I do think our full-backs have been high and wide and we’ve been open.
‘Matt Clarke coming out as well has meant two of our back four have been up and around their box on a regular basis.
‘If we’re doing all that and keeping clean sheets then I’m fine – but we’re not.
‘It has to swing towards making sure our defenders can defend and keep clean sheets, our midfielders can compete and the ball runs through them and then the front three get a good service to score and provide assists.
‘We’ve been slightly too open and it’s been too easy for teams to sit back and counter.’
Pompey finished the season on 88 points – three short of what was required for automatic promotion.
Having shipped three against Peterborough on Tuesday and another one against Accrington, Jackett knows changes must be made.
The manager added: ‘We can’t necessarily say there’s too much wrong by getting 88 points.
‘It would have been lovely to have got another four or five but that’s the way it is.
‘Thirteen away wins is a club record, so we’ve done well.
‘But at home we haven’t kept enough clean sheets and maybe that’s been some of the reason (why Pompey missed out on automatic promotion).
‘Certainly it has been this week, conceding three against Peterborough and one on Saturday.’