Where is the lever. That usually starts happening when the locking bolt needs to be rebuilt or replaced. It starts sticking.
If I remember right the KX-5 came out in 2002 as an upgrade to the KS-5 Special. They changed the forend release to a rod and end button similar to a Perazzi TM-1, from the under side release lever, and the went to an adjustable LOP trigger with a wider comfortable blade like the K-80. They added a forend iron block also so you can see it, instead of a steel insert in the wood. They all came with the adjustable rib with the dial adjustment. I believe all were choke tubed and they had an adjustable comb. They also went from a slanted post rib to a straight up post. The changes were I guess an upgrade, but maybe also a marketing change.
Otherwise everything else was pretty much KS-5 Special. The receivers are identical with parts and configuration of the same simplified but very efficient construction.