why disproportionation of free radicals does not happen in kolbe's electrolysis if we use secondary or tertiary carboxylic salt as substrate?
Because of the free-radical-scavenging activity of the secondary/tertiary  carboxylic salts?

Hence, radicals that could possibly undergo disproportionation reactions resulting in alkene are removed.

As a possible example, see 'Evaluation of the Free-Radical-Scavenging Activity of Diclofenac Acid on the Free-Radical-Induced Haemolysis of Human Erythrocytes'.