Originally Posted by Iceblossom

Some days just don't go well... the other thing I'd look at if you continue to have problems is if you have to adjust the foot height. With my friend's set up we had to move the foot depending on whether we were using her warm and natural or my poly bolt, and sometimes the foot would just slip a bit. As I recall, with that set up a dime was supposed to be able to move freely under the foot at normal setting.

And keep in mind that even a brand new needle can have a flaw in it, so if you have yet another problem, I'd do a complete rethreading, new needle, and maybe try a different bobbin too.