Question, what is the wire feeding like? Is it intermittent? Jerky? Drive roller alignment? Drive roller pressure? Liner plugging, wire spool brake too tight?
We work on 14' cylinders, and the ground is attached via two large carbon blocks (like massive brushes on an electric motor) mounted opposite of the electrode (spaced about ~45° apart). For sure, there are multiple work leads...6 in fact.
We even went so far as to remove the carbon arc blocks and set up the old solid copper Work-load clamp (The kind ya bolt down with a 3' pipe wrench.
All very good items to consider.
Thankfully, we have identical 'lines' here, and so we moved our machine next door, hooked it up to the other line, and the issue persisted. With that, our feeling that the ground connection was the issue resided not in the grounds/sense lead.
The awesome help at Lincoln even helped us check the voltage, amperage, etc. etc. at the studs on the power supplies. That's some phenomenal diagnostic tools.