My Vertex HV-6" is smooth working. 1 turn of my RT handle moves the table 4 degrees. I just checked it. My Vertex is well made and machined. If it matters, Vertex is manufactured in Taiwan. Penn Tools has a link to Vertex Machinery.
https://www.penntoolco.com/vertex-machinery-and-tools/ Penn Tools seems to think they are pretty decent.
Yours may need adjustment, cleaning and disassembly or replacement. Vevor is not exactly known as a top quality supplier. I know, I have a Vevor piece of equipment. Mine has every possible cost reduction possible in it. It works, but is not refined or smooth in any way.