It's a tension/thread quality issue if the 301 in my opinion1. Can be readjusted from the bobbin tension upwards if that's the main thread you plan to use. I found the 221 (same issues) way too fussy with tension issues going between overlock thread to the likes of Guttermann so that was left alone. I do use overlock thread for most of my construction though but that is on a 15-91 & 401A which have no problems with the stuff.
ps: set bobbin tension to allow for the overlock thread and then set the upper tension to balance the bottom
