Sealed Bottom bracket won't screw in
Sealed bearing UN55 BB. Left (fixed, non-drive side) screwed in just fine.
Right (drive side) just doesn't screw in, and I know it's a left hand (counterclockwise) screw.
The old hardware threads just fine. Threads not stripped.
Diameter and thread size measure OK. Item not mislabeled.
What the heck am I doing wrong??