Hi James,
As Peter commented, your blurry stacking might be due to a nearby star. I had no problems using the comet stacking workflow in the DSS User Guide for my C/2020 F3 (NEOWISE) series of exposures
https://britastro.org/observations/observation.php?id=20200716_224500_72d8be81be01185c
I zoomed in to each image and click / registered the bright centre of the coma.
The sky motion of the comet will also be a factor, which is more evident in narrow field images rather than wide-angle views. The exposure duration and number of images in the stack can exaggerate the trailing. For my recent images of C/2022 E3 (ZTF) I took 20 x 4s exposures through my C11 and just used the default ‘star field stacking’ in DSS.
Alex.