if you thread by cutting the thread, adding new & pulling it through...sometimes the 'new' thread does not make it through the tension disks completely- its important to pull on both sides of the thread around the disks & make sure the thread is in there correctly- this can make such a difference= you 'know' everything is done right...but the thread is actually laying on top of the tension disk's instead of being between them-so not getting the tension correct on top---which would cause problems on the bottom.