While there were some encouraging signs in terms of performance in Saturday's 3-2 defeat against Celtic in Glasgow, Stack concedes he and his team-mates find themselves struggling to pull themselves out of the downward spiral which has seen them win just two of their last 14 matches.
"Once you get in a rut like this, it becomes harder to turn things around the longer it goes on," said Stack.
"If you could put your finger on what has changed with us this season, then you'd be the best manager in the world. There are a number of things letting us down at the moment and we have spoken about them within the club.
"We know we haven't worked hard enough. We know we are a shadow of the team we were."
Stack remains hopeful, however, Hibs can salvage something positive from their campaign in their remaining four SPL fixtures as they look to hold onto fourth place in the table which will guarantee European qualification.
"There is no-one in this team at the moment who is not going to roll up their sleeves to try and turn things around," he added. "We are not feeling sorry for ourselves, we are genuinely trying to get back to winning ways.
"Looking in from the outside, I think people would have seen glimpses of character and passion to try and win against Celtic. Unfortunately we just didn't show enough nerve or bottle to hang on for at least a point.
"For us to concede a third goal was devastating. We had put so much into the game for 80 minutes. I thought it was massive improvement on the week before against Hamilton, but then it couldn't have got much worse in fairness.
"I'd like to think this was a step in the right direction. All right, we have been beaten 3-2, but there were a lot of positives from the way we competed. We have to lift our heads and the last four games are huge for us."