Kenny Jackett has pledged to eradicate the Pompey vulnerability which is threatening to devastate their season.
The Blues were cruelly denied victory over Oxford United on Saturday by Matty Taylor's last-gasp leveller.
It represented the third-successive weekend Jackett’s side have dropped points after conceding in 90-plus minutes of a match.
Overall this season, Pompey have now relinquished eight League One points through letting in goals during the final 10 minutes.
It’s an infuriating statistic which is presently condemning the promotion hopefuls to 15th in the table.
And, following the 1-1 draw with Oxford, Jackett is again challenged to come up with a solution.
He said: ‘We must persevere, keep going, it’s a frustrating thing, we have worked hard to get ourselves into a number of good positions this season and again on Saturday couldn’t close it out.
‘We have to keep working hard at it, identify what we can do in those closing minutes to be able to get tighter, such as block the cross and ultimately be able to mark the centre-forward on the far post.
‘Maybe it has become a mental issue, but that’s the way it is, we have to work hard to come through it and we have an honest group of players.
‘I do think, in terms of standards, it has gone up pretty well in the last two games and that's a good thing, but also we’ve got two points out of six.
‘We want six points out of six – and seeing games out is a big part of it.
‘You have to say, we either should have been out of sight or closed it off, that’s it. We can’t be happy with where we are and drawing on Saturday.
‘It wasn’t an even game necessarily, we were on top, and had a lot of chances as well, but we have let a few leads slip now – too many in terms of getting ourselves up the table and into a good league position.’
Pompey had been leading 1-0 through Gareth Evans’ 58th-minute penalty heading into the final minute.
And, as at AFC Wimbledon and Bristol Rovers in previous weeks, a late leveller deprived them of points.
Jackett added: ‘Tactically, did we cross once or twice too many times from the corner flag position? Then, on the break, did we do enough, were we clever enough?
‘Even so, we still had enough players back, it wasn’t like we were outnumbered. It was a cross on the right touchline that has gone right over everyone and been headed in from beyond the far post.
‘It is a real issue and something we have to come through.’