I received some replacement bulbs for my 328k and I couldn't get them to twist and lock. I asked my husband to help me screw in a lightbulb and he thought I was making a joke about the old "How many people does it take to screw in a lightbulb? " He gave it a whirl and couldn't do it either. I emailed the Sew-Classic people inquiring about the issue and meanwhile i dug the old bulb out of the trash to compare. The button thing on the bulb that locks it into place is different. The new bulb has wider buttons. My husband took a file to one of the new ones and made the buttons more narrow and now it screws in. I thought I would share in case someone else had the same problem.
ETA: In the picture the new bulb is on the left.