Albion's second Group C clash in the Premier League Cup ended in a cruel 2-1 defeat against Swansea City at The Hawthorns on Thursday night.
Ollie Bostock equalised for the hosts with 15 minutes remaining after an unfortunate Ben Cisse own goal had handed the Welsh side a slender advantage at the break.
The Baggies had the lions' share of possession and chances, but were hit with a deadly blow in second-half stoppage time when Aimar Govea won it - tapping into an empty net having initially hit the post.
The setback leaves the club's young hopefuls on one point from two matches.
The hosts had the first opportunity of the night. Harry Whitwell made a powerful drive into the penalty area inside the opening ten minutes and unleashed an effort at goal, however Swansea goalkeeper Ben Hughes made a smart save down low to his left to deny the midfielder.
Less than 120 seconds later, the ball was in Albion's net. A surge forward on the left wing from Govea resulted in his cross being headed against the post by Baggies skipper Muhamed Diomande, only for it to then hit stopper Ben Cisse and trickle into the corner.
Richard Beale's boys controlled proceedings after that moment of misfortune and created plenty of chances to draw level before the break.
Fenton Heard and Deago Nelson both saw strikes from the edge of the area fly just over the bar, while Nelson and Cole Deeming had long-range shots blocked by Swansea's defence.
Bostock's teasing cross was also nodded over by Heard from 12 yards in what proved to be Albion's final opportunity of the first 45.
The lads wearing navy blue and white stripes started strongly after the break and almost equalised when forward Eseosa Sule's deflected shot from the right side of the box struck the outside of post and went behind for a corner.
Whitwell then had two chances to score just minutes after the hour mark. Heard's clever free-kick slipped the 18-year-old in behind, only for Harry to be thwarted by Hughes between the sticks, before volleying wide after Swansea had failed to clear their lines.
Bostock was next to try his luck, forcing the Swans stopper into a save down low. Moments later, Heard headed straight at the No.1 following an eye-catching cross from the left from Whitwell.
The pressure finally told thought with 15 minutes remaining. The Baggies broke through the middle of the pitch and Whitwell found Wales youth international Bostock free on the left side of the penalty area. The winger wasted no time, slamming home into the far corner at the first time of asking to make it 1-1.
Albion were doing all the attacking, but left themselves a little short at the back in the final stages of added time. The ball broke kindly for Govea and after seeing his first attempt come back of the post, the No.7 was on hand to stroke into the middle of the unguarded net.
ALBION: Cisse, Nelson, Diomande (C), Shaw, Frabotta (Parker 58), Iddrisa (Richards 58), Deeming, Whitwell, Heard (Higgins 76), Sule (Chimeziri 58), Bostock.
Unused subs: Wallis.
