Pompey Academy saw their recent run of good form ended by a 3-0 Youth Alliance League defeat at Exeter.
Jay Stansfield and Nelson Iseguan both capitalised on set-pieces to give the hosts a comfortable advantage at the break.
The Blues then saw Liam Kelly controversially sent off before Cheick Diabate netted on the break to secure victory for the Grecians.
Academy manager Mark Kelly said: “We beat them 4-1 at home a couple of months ago and expected a high press, knowing that we needed plenty of energy.
“But we tried to play short when we should have gone long more and once again defending was a problem for us. It’s becoming a common theme.
“They scored from two set-pieces in the first half and we were simply not good enough at the back stick.
"Once again defending was a problem for us. It’s becoming a common theme."
“We’ve been a bit better in recent weeks, so it was disappointing to see us slip back again and it’s clearly something that needs to be picked up on.
“After the break we were more free-flowing and our substitutes provided a bit more energy, but fair play to Exeter because they defended for their lives.
“Liam was sent off, apparently for something he said after a clash with one of their players. Although after speaking to their coaching team, it doesn’t appear that he said it, so we’ll have a look at that.
“We were on the attack and they score on the break – again it was poor from us and at that point it’s game over.
“We’re still conceding too many goals and it’s clear that our defending needs to improve as a unit – I’m not just talking about the back four.”
Pompey: Pitman; Bruce, Robb, Mnoga, Kelly; Bridgman, Read; Maloney (Kavanagh 65), Whiting, Flint (Johnston 62); Stanley
Sub not used: Brook