The first thing to check when there are starting problems is the battery ground. Often the ground corrodes or doesn't have a good connection to the engine.
Since your tractor is new, perhaps a cable is not secured. Check all the positive and negative connection points from the battery to the frame and to the starter to make sure they are tight.
If that doesn't make a difference then try taking the ground cable off the battery and connect the battery ground directly to the engine on a bare spot with a jumper cable. See how the starting performance is then.
If it is better, then buy a long battery ground cable and attach it to a bolt on the engine. This will essentially connect it directly to the starter.
I had this problem on two tractors in the past but they were older.