What is the problem with forming 2,3-dimethylcyclohex-2-en-1-one? I have suggested a mechanism for the formation of 3-ethyl-2-methylcyclohex-2-en-1-one A 



but the question says problems would be encountered if a similar route was used to make 2,3-dimethylcyclohex-2-en-1-one B. I think perhaps a decarboxylation step is involved? Can someone please explain how to get to B and the difficulties in the mechanism?
Your starting product for A is symmetrical, but your starting product for B will be asymmetrical - one carbon less on one side.  This is going to mean you may get 2 or more different products; B may not be your major product, and you'll have to separate them afterwards.  I don't believe there's inherently a tricky difficultly in the mechanism for B.