I literally do not even know what your tractor looks like, but I will tell you what I experienced on my M6800. The same basic thing was happening. Sometimes took off and worked perfect. Other times nothing. If I put the shuttle into gear with a little more force, it would go. Other times, no. Look for an adjustment down below the handle. I'm sure somebody will jump me because I don't know what I'm talking about with your tractor, but my tractor had a threaded sleeve with 2 outside retaining bolts on the frame just down below the shuttle handle. I took those 2 bolts out and the sleeve allowed the lever to go in farther and my problem was instantly fixed.
And DO NOT just assume that the dealership would have found such a simple thing as that adjustment. Over time, I walked into three different dealerships and asked what was wrong. They would hint toward the handle needing adjustment, but nobody could tell me where that adjuster was. I finally found it on my own.