What could account for less than 100 percent yield of co2 from your chemical reaction?

A number of factors could be responsible for getting less than 100% yield in your reaction. It could be due to any of the following:

1. the reaction may not produce a stoichiometric amount of product due to equilibrium, slow rate, decomposition of reactant or product, evaporation, etc.

2. the reactants may not be 100% pure

3. Undesirable side reactions may have taken place.