(A) If am correctly interpreting the "subject matter" here (variously called "kak", "clam", "split Note", we are perhaps talking about the most ubiquitous problem in all of Brass playing! The easier part is to describe some of the many things which can "go wrong". Perhaps the more difficult part is to determine which factor, or more likely, combination of factors, are responsible for that last "clam". I view the "split note" to be the result of a MISMATCH. A logistical error, if you will, involving your perception of how the note "should" sound, and the physical "focus" (meaning, the various parameters which comprise the embouchure) which the player "selects" (as opposed to the one which is REQUIRED) in order to achieve the tone in question. A pure, clean, precise tone is dependent upon the coordination of : correct (appropriate) use of the tongue (attack), the correct "buzz" (lip tention), and the correct amount of air pressure. As Mr. xxxxx correctly pointed out in his response, the first opportunity for error lies with the player's incorrect mental/aural intrepretation of the written note. "If you can't "hear it", you aren't going to be able to play it!
"Clams" are often simply the result of :(1) Inadequate warm-up, ie., the "lip", and attending factors, is/are not yet adequately responding to our demands. (2) At the other end of the spectrum lies fatigue - which prevents us from "getting to" or maintaining the correct "focus". (3) Music whose demands are simply too acrobatic for our skill level. Inability to quickly adjust the "focus" of the embouchure. (Flexibility problems) (4) A problem, (though admittedly not high on the list) can also occur with an overzealous application of the lip/jaw vibrato - "purposefully" disturbing the embouchure. (5) As we approach our register "ceiling", be it high or low, our percentages of "mismatches" will increase. (6) And finally, lack of attention, early on, to the FUNDAMENTAL principles of tone production (attack and release), can result in increased difficulties. (7) LACK OF CONCENTRATION will take it's toll in a heartbeat! In a nutshell, I would define the problem as "failure to be in the right place, at the right time"! Thanks for listening, Clyde Hunt