I sometimes have voids in a composite core that I notice after preparing the core/tooth for a crown. Do you have any recommendations to help with this problem?

You might try a more flowable core material. FormCore DC (J. Morita USA, Inc.) is a flowable composite that is dispensed with an auto-mix mini-syringe directly into the preparation. Because it is flowable, a matrix is recommended to contain the material during placement. Place the core material in 1- to 2-mm increments and light-cure each increment.