Using Rope to lift an object

[d4rk3lf]

What should I change to give the rope more "strength"?

Try increasing Max binds parameters in PhysX Bind operator, to some larger number. 
Also, you can try changing mass parameter in the PhysX Shape operator (for the ropes), to something larger then 1, so the ropes have more influence on the logs. 
And last but not least, grab your Tyflow Icon, and increase Time Step to something like 1/4, or 1/6. This will make it slower, but will globally influence all PhysX behavior to be more stable (as it will calculate motion in between frames). 
And also, there is separate substeps option under the PhysX parameters, also in Tyflow icon, that will globally change the physX behaviour. 

Btw, I've tried now cloning log to 4 logs in my file, and it worked without problem. No stretching on the ropes, and it lifted all 4 logs. 
Maybe when you replicated my scene, you missed some of the settings I mentioned above (probably max binds) 

Keep in mind that PhysX Shape operator can't work with concave shapes, only convex. 
There is a workaround, by braking concave shape to smaller convexe, then binding them back together. 
But it's not that easy to explain.