Town's U18s suffered a 2-0 home derby defeat to Norwich City at a rain-drenched Playford Road on Saturday morning.
The young Blues were without centre-back Tommy Smith, who was ill, and striker Jordan Rhodes, who has a back injury.
In a quiet first half, both sides created half-chances but neither keeper had to make an important save.
Right at the end of the half, Norwich keeper Steven Arnold looked in trouble after mis-controlling a ball outside his area and Jack Ainsley passed the ball towards the open net, but referee Sandell contentiously blew for half-time as the ball was rolling towards the goal.
The Canaries took the lead on the hour mark. A header from a corner was only clawed away by Plummer and centre-back Alvin Quaye headed the ball into the roof of the net, over Kurt Robinson who was on the post.
It was 2-0 three minutes later when a linesman adjudged that another header from a corner had crossed the line, despite Ed Upson appearing to have cleared the ball off the line. The Town players were furious with the decision and Russell Short was booked for dissent.
With ten minutes remaining, it could have been three when a Town corner was cleared and, after a good through ball, Bally Smart was left one-on-one with Plummer, but the young South African striker struck the outside of the far post.
Town failed to create many chances during the game and looked fragile at the back without Smith, but Upson and Reason were impressive in midfield.
Meanwhile, Town's U16s also suffered a defeat to their local rivals, losing 3-1. Craig Calver scored the Town goal prior to coming on for the U18s.
Town U18s: Plummer, Short, Robinson, Upson, C Williams, Fenn (c), Nimmo, Reason, Leabon (Calver 78), Ainsley (Barrett 54) and Tooms (Boto 69). Unused: Clemo.