Leafs rank 3rd in xgf/60 on the PP, and were 1st for a while before this recent cold snap.
The biggest problem is definitely that Tavares isn't converting anymore. Since he's getting the bulk of the slot chances his expected shooting percentage is the highest on the top unit, but his actual shooting percentage is the lowest.
Using unblocked shot attempts percentage, not just shooting percentage
Player: Unblovked Shots - Expected % - Actual %
Tavares: 69 - 15.1 - 5.8
Nylander: 70 - 8.9 - 11.8
Matthews: 51 - 12.7 - 24.2
Marner: 23 - 13.0 - 23.8
Rielly: 18 - 8.6 - 9.0
So all of them hitting at significantly better than expected shooting percentage other than Tavares, who is way, way below his expected rates.