Pompey slipped to a fourth successive defeat, going down 2-0 to West Ham at the Wellington Sports Ground.
Daniel Purdy opened the scoring for the Londoners midway through the first half, completing a fine move by finishing past Tom Fry.
And England youth international Robert Hall sealed victory from the penalty spot after the interval.
Director of youth operations Paul Smalley said: “There were lots of positives to take from the game despite the disappointment of another defeat.
“All of the players must reflect on their individual performances and try to reach a level of consistency that will bring more positive results.
“It is a frustrating time for us currently. We’re performing well, creating chances and enjoying a lot of possession but then a refereeing decision, lack of concentration or mistake changes the nature of the game.
“The first half of the season has been a massive learning curve for all of the players.
“We will be working hard on our performances after Christmas with the players coming to terms with the demands of being part of a professional football club academy.
“Most of the players – even the younger ones – have had some great experiences with game time and even opportunities in the reserve friendlies.
“We also have our FA Youth Cup game at Manchester United on December 6 to look forward to. That will be a great experience for all and hopefully we can repeat last season’s impressive run in the competition.”
Pompey: Fry; Tallack, Bennett, Magri, Grant; Seeley (Antelmi 56), Stockford (c) (Higgins 56), Tsovolos, Butler (Colson 72); Walshe, Williams
West Ham: Larkins; Modelski (Young 80), Lee, Hunt, Driver (Potts 72); Wearen, Powell (Hurley 67); Hall, Moncur, Purdy; Subuola