All NH (as well as anybody else's square balers) work fine with plastic twine if the twine disk tension is adjusted (increased) to retain the twine ends. Same for pullout force from the twine box and billhook jaw retention. Both should be about 10 lbs and measured with a fish scale. Just like a manual describes.Only thing I disagree with. Older as in ancient NH bailers aren't designed or built for poly, only Sisal twine as the twine discs cannot retain poly, it's too slippery and the bill hooks aren't hardened either and will quickly wear with poly, if you can even get it to tie at all. Far as the bale bunching up, when bailing wheat straw the tension has to be kept high, not loose and he probably needs 'hay dogs' in the sides of the bale chamber past where the plunger extends. they keep the forming bale from backing up and NH sells them for all models both ancient and new. I used to bale wheat straw in small squares on contract, thousands of them but I sold my square bailer a couple years ago, I just do large rounds now. I had a well maintained NH 575 high capacity with the extra sweep pickup and hydraulic bale tension and hydraulic tongue swing and I hope the OP is swinging the bailer off to the right side when using it. I got 14 grand for it in 2 days flat, cash sale and the buyer contracted it to be hauled and the permits too as they are over width and the tongue cannot be easily removed. No CL or Farcebook Market place either. I have zero patience with tire kickers and low ballers. Sold it on Tractor House and paid a 200 buck insertion fee with multiple pictures in color of course. Just sold a Kuhn Master Drive double rotor rotary rake pull behind as well and bought a new Kuhn Masterdrive SINGLE rotor 3 point hitch mount and broke even. I've reduced the acreage I cut and bale and the twin rotor was just too big and I wanted a 3 point anyway. Easier to square up windrows on odd shaped fields.
What I often see is operators attempting to use round bale twine in square balers because its cheaper and longer length overall, so they think they are getting a deal. The round bale twine is much thinner and more difficult to retain in the twine disks. But, I've seen it done successfully. Round bale twine is not as strong as square bale twine because there are many more than 2 strings involved in the process, so they wind up complaining about bales breaking during elevator trips.
There is no physical reason that plastic (BT-130, 10,000ft) can't work instead, otherwise all of our shoes would be falling off. You wrap a bale being formed, you bring the ends together in 1 place, bill hook opens and captures the 2 strings, it twists them into a bundle, the wiper arm scrapes the bundle off, and the result is a tied knot.
Believe it or knot, some actually believe that the COLOR of the twine must be factory spec'd, too. Lore from round bale or square bale purpose, plus the 'natural' sisal was made from hemp. My place used to be a hemp farm during WW-II, delivered to the US Navy for rope (lines). I like any color that you can see when grabbing them by hand for delivery.