cats who were not separated too early from their mother find having their scruff pulled to be comforting, since this is how their mother carried them when they were young. Techs usually "scruff" cats when they are being examined for this reason; it calms them down so much that they are less likely to resist being handled.
However, now that she's an adult she's far too heavy to be supported by the scruff of her neck. Adult cats are very bottom heavy, and as such their rears need to be supported if they are being held or it will be painful for them. If you really want to hold her but she doesn't want to be held, you can try holding her properly (supporting her butt) and using a free hand to scruff her. When she's on the floor/bed/whatever, grab her scruff and gently pull it, and while she's scruffed, scoop her butt up with the other arm.
vet tech, cat owner