Washington has a no nonsense approach to permit reciprocity. Their state law (RCW 9.41.073) states that Washington will honor permits issued by states that meet the following three conditions:
- Recognize Washington concealed pistol licenses;
- Not issue concealed pistol licenses to persons under age 21;
- Require a mandatory fingerprint-based background check for criminal and mental health history.
On June 2nd, 2015 the Attorney General's office updated their official list based on that criteria and in doing so stopped honoring permits from Arkansas, Kansas, Mississippi, and Missouri. You can find the official site and list here:
The individual reasons for each of these states are as follows:
Arkansas: Allows persons under 21-yrs-old to have license.
Kansas: Does not recognize WA CPL in a meaningful way as will only recognize if holder is KS resident
Mississippi: Allows persons under 21-yrs-old to have license
Missouri: Allows persons under 21-yrs-old to have license