How to create a seamless pattern from a seamless tile?

[ss-kalm]

You're definitely scaling something wrongly.

The base 2" x 2" tile is okay ... It doesn't seem to have a line width, although it does have a grey line on the left hand side, which comes through in the repeat.

Scaling this up to 20" x 20", you should get a 10 x 10 repeat so something is really screwy with your tiling/scaling. If you're resizing the base 2" x 2", then your resize is off somewhere.

For me it repeats like this at 8" x 8"