Lappin – 32 on Saturday – was expected to complete the full session but will be assessed by staff on Thursday after failing to finish.
There was speculation Lappin had suffered an ankle injury.
He had been earmarked to make a welcome return this weekend after being sidelined for eight weeks with injury.
Lappin, who missed the entire 2006 season with an ankle injury, was expected to be eased through the reserves rather than the Lions' clash with Adelaide at the Gabba on Saturday night.
The veteran midfielder had hit the ground running this season, averaging 18 disposals in the first four rounds before disaster struck.
A question mark loomed over not only the three-time premiership player's season but his career as he battled to overcome achilles, heel and foot injuries.
But there was light at the end of the tunnel for Lappin when he did extra running work last week and joined the reserves for training last Saturday without showing any ill-effects.
But the setback has further delayed Lappin's return.
The Lions' injury list also includes key forward Daniel Bradshaw (hamstring), who is doubtful for the Crows clash.
But Jason Roe (hamstring), Michael Rischitelli (hip) and Scott Harding (knee) are expected to be available.
Lions utility Jared Brennan said the whole team could be placed on the injury list after last round's 63-point loss to the Western Bulldogs, suffering from damaged pride.
And there is no let-up in sight.
Still smarting from losing "respect" against the Bulldogs, the Lions don't have to be told they have to shape up before facing a near-carbon copy of their last round tormentors on Saturday night.
"We did lose a bit of respect [last round] and Adelaide is just as good as the Western Bulldogs," Brennan said.
"Adelaide Crows play a similar style to the Western Bulldogs as well.
"They get a lot of numbers behind the ball and run from there.
"It will be imperative we stop their run from behind so they can't link up with handballs."
The Lions were flying with four straight wins when they arrived at the MCG to take on the Dogs last weekend.
It was seen as a litmus test for the Lions, who are finally emerging from the lean post-premiership rebuilding years.
"It was a massive reality check," Brennan said of the loss.
"Inside the group we probably thought the finals are a long, long way away but we know we didn't play to our best capability – we were terrible."
Brisbane has lost their last three games against Adelaide, including two at home.
