You have a low voltage problem.
I had the same thing on my John Deere 350D and it took awhile to find too, so don't feel too bad. I always start with the easiest and the least expensive things to check first. Here is a short list...
1. Check to make sure the positive lead wire is not chaffed and shorting out on the chassis somewhere robbing your starter of voltage
2. Check batteries
3. Check negative battery terminals for contacts on chassis
4. If you have a spare battery cable, try attaching it from starter to chassis, sometimes a starter does not get a good ground through the chassis due to oil and paint, etc
5. But, you said the tractor is industrial. I wonder if it has a battery disconnect switch? If it does, that is most likely your culprit. That is NOT a knife switch, it has contacts that as you move the lever, it brings power upwards and into contacts. These arc out over time and cause low voltage problems. It took me awhile to find that issue on my tractor.
It is best to put the disconnect switch on the negative side of the batteries, but is not always easy to do. It does however prevent arcing inside the switch.