I think TZbrown is right on the hook distance. Your hook looks good if a little dull but not close enough to the needle. With the looser weave of webbing you get a little more forgiveness but your poly might be closing the curtain too much. Also the tensioner spring could effect the thread slipping off the hook. I always tell people that sewing machine repair is a black art, not a science. What works on one machine isn't always the case on another.
Just keep trying, everything looks functional and it's very likely just one or two things are throwing you off.