Brisbane led by 22 points early in the third term but only clung to victory after the Crows rallied in a 15.9 (99) to 13.15 (93) rollercoaster ride on Saturday.
Harry Schoenberg's snap on the run breathed life into the visitor's fourth-quarter charge before Josh Rachele hit the post and Schoenberg had another set shot fall short.
The ball was locked in Brisbane's defensive 50m for the final two minutes but the Lions held on, moving to 15-6 and keeping hopes of a top-two finish alive and locking in a top-four berth.
With two rounds to play the Crows (10-11) will need results to go their way to crack the top eight.
Ben Keays (19 disposals, two goals, four score assists) was immense against his former team, his relentless running setting the tone as they went toe-to-toe like few sides have managed at the Gabba in recent seasons.
"Very, very disappointed," Nicks said.
"We've got all the ingredients there and we're not getting it done.
"We want to be a great team, at the moment we're a very good team taking it up to the best.
"Key moments in the game; had our chances and didn't take them."
Keays had his second early in the third quarter when he was again first to the contest while Rachele enjoyed his snapped major after being substituted on in a move the coach stressed was a "positive" one.
The 33-year-old Walker (two goals, three behinds) took his season tally to a career-high 65 goals while Shane McAdam kicked four goals.
Brisbane had Charlie Cameron's manic third quarter to thank, the small forward booting three and setting up another two.
He expertly guided through Lincoln McCarthy's squeezed shot, then knifed a brilliant, low cross-kick into Cameron Rayner's path for an easy goal.
He missed a shot for his fourth goal as the siren sounded on a back-and-forth quarter that, thanks to his efforts, ended with the Lions up by 16 points.
Fagan said a six-day turnaround after a tough defeat of Fremantle in Perth looked to have taken its toll, but saved plenty of praise for the Crows.
"I felt like we were slow to start and looked tired at the end and I don't know whether that explains it or not," he said.
"Because in between ... that looked like us."Â
Fagan said Brisbane's veteran forward Jack Gunston, who was substituted off after twisting his knee in a tackle, will likely miss the rest of the season with a suspected MCL injury.
The Hawthorn recruit is contracted for next season, Fagan confident it wouldn't be the 31-year-old's last AFL appearance.
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