I agree with Graham's initial post, however I would like to add to it.
The issue with card stock usually occurs with the feed rollers (usually rubber rollers or roller wheels). These rollers get a glazing on them that does not grip the paper properly. They will appear to be clean, but the glazing is still there. If you clean
them with a soft cloth and alcohol, they should work properly. If the feed rollers are shiny, they will not grip properly.
Another issue if you are using a printer that feeds the paper in a semicircular path (feed is into the machine then comes back out the same way it went in), is friction on the paper itself due to the stiffness of the card stock. Check the paper path to
make sure it is clean and free of anything stickey that would slow down the paper.
Watch the paper when the printing starts, if it makes the feed sounds, but the paper lags, the start of the printing, it is slipping. As a alst resort, if you have a limited amount of printing to do, you can always apply a slight "push" to the paer to get
it started. This is not the best solution, buts it does work in some cases.
Good Luck.