Some of the returning players are the problem. They've been in the league for a few years, they've won a Kelly Cup (or two) and know by now they're not going to move up to play in the AHL and/or earn an NHL contract. That opportunity is now behind them and they're not as motivated to go out and compete every night the way they did their first two or three years pro because they no longer see a reason to. They're no longer young (by pro hockey standards) and hungry.
I'm not seeing lack of effort. I'm seeing defenders trying to stand up at the blue line and getting beat (over and over again), dump ins on power plays that come right back out (over and over again), and players consistently out of position in their own one allowing easy scoring chances.
Looks like system doesn't match skills to me. Not effort.