TC converter

Edit My Images
Who uses a 1.4 tc with no issues?
I have a Nikon D800 and a Nikon D7200.
For wildlife I use a sigma 150-600 sport.
I liked the D7200 for the extra reach with the crop factor but the D800 gave better results especially in low light high iso situations.
Even cropping the D800 image was still better than a D7200 image.
So I thought I'd get the Sigma TC 1401 1.4tc.
On the D7200 with the TC autofocus works fine. Even in iffy light.
In the D800 it's unusable. Hunting, slow never locking on etc.
I get better looking images by severely cropping a D800 image without the tc than using the tc with the D7200. Which means the tc was a complete waste of money yet I have seen some images with a tc that are superb.
Anyone else had similar issues