Hi Hattie
I have this model and the 6268. I take mine to a sewing machine sales/repair place for tune ups at least once a year. The bobbin area in these are quite picky about lint build up and such. The bobbin style in this machine does not have the center hole like most bobbins.
http://www.sewingpartsonline.com/bob...-312956-s.aspx
See this link for the bobbin style for this machine. They also sell a generic brand but I have not had any good luck with them. They are spring loaded centers. If the spring is worn out, you need to get new bobbins. Once you see them you can see why you can not wind bobbins for these machines on a separate bobbin winder.
When winding your bobbin you need to make sure the slide is set just right on the mark correctly or else it will wind underneath and not on the bobbin itself. First I would try a new bobbin. Check if the spring is working right. If you can not get it to wind properly after attempting several different bobbins, try taking the machine to a sewing sales/repair place to have them look at it for you and maybe have them clean it if the charge is reasonable. I hope this works out for you.