After conceding triple-figure scores just four times in each of their last two flag-winning campaigns, Brisbane (3-3) have already done so three times in just six games this season.
The latest example resulted in a shock two-point defeat to Melbourne despite the Lions breaking through the 100-point barrier themselves.
Fagan has identified the leaking as the key issue facing his team as they prepare to take on a potent Adelaide attack at the Gabba on Sunday.
"We played well on offence (against Melbourne) and we played well in the contest, but we played poorly on defence, so there's a big focus on that this week," Fagan said.
"It's not just (tackling), it's the whole system, and it's been the case in a couple of games this year.
"The Bulldogs game at the start of the year and that game on the weekend ... those two, in particular, where we kick 100 points but we lose, that just says there's something wrong with your defence.
"We'll go to work on that this week."
The Lions' back six has been hit hard by injuries with Darcy Gardiner (shoulder), Dayne Zorko (calf) and Jack Payne (knee) among those sidelined.
Fagan has further personnel issues this week as he needs to replace lockdown defender Noah Answerth (concussion).
The 26-year-old sustained his second concussion of the season in the loss to Melbourne, but has showed positive signs this week.
"He's really good actually, which I'm pleased about. I was worried for him," Fagan said.
"He was really bright yesterday, so that's a good sign. Sometimes they get worse two or three days later.
"Judging by his response to the weekend, I would say it hasn't been a bad blow to him."
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