The USSR was not invited to the Munich "conference", which was a big mistake if you consider that in 1939 the Western Allies tried to make negotiations with Stalin. So Chamberlain was not discussing the European politics with Stalin in 1938, I think.

In 1938 it is perhaps that Chamberlain partly believed he could do the work himself and get Hitler to agree to a deal. Also the USSR as part of its help proposal required "pathways" through countries to help the Czechs which for countries like Poland was not ok. So Stalin was left out of the negotiations for several reasons.

This was rather interesting as well:

CJO - Abstract - THE SOVIET COMMISSARIAT OF FOREIGN AFFAIRS AND THE CZECHOSLOVAKIAN CRISIS IN 1938: NEW MATERIAL FROM THE SOVIET ARCHIVES

The exchanges between Litvinov in Moscow and the Soviet polpred, Alexandrovsky, in Prague show clearly that the latter was repeatedly cautioned against encouraging the Czech leaders to think that they could rely on the unilateral assistance of the USSR. They reveal, too, the degree to which Litvinov and Potemkin, a deputy commissar, felt that Fierlinger, the Czech minister in Moscow, was misrepresenting the Soviet position in this respect. Additional evidence cited here confirms earlier views that the Soviet leadership was not prepared to act independently of France or outside the League of Nations even when the opportunities for assisting Czechoslovakia were available.

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