Slow Row/Bench Progress - by: Dee

I'm currently 140 lbs and I follow a full-body routine called the Fierce 5 from bodybuilding.com. I've definitely gained weight and strength but progression on my bench and pendlay is slow as fuck.

Squat - 165 3x5
Deadlift - 160 3x8
OHP - 65 3x5
Bench - 125 3x6
Pendlay Row - 120 3x6

Question 1: How much of a weight difference should there be between my upper-body and lower-body lifts.
Question 2: I can only do 3x6 120 Pendlay rows with good form before it breaks down. Should I incorporate single DBs?
Question 3: Why are my back and chest stalling/slow as fuck? Should I add more volume?