Pure salt cannot lose its savor (or "saltness"), but the salt commonly used in the ancient world was rock salt, containing various impurities. As the true salt was leached away, or otherwise removed, the so-called "salt" could indeed lose its savor.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8