depending on the age of the kit, you might find an issue with the transition for the 'payload' section, some of the early kits had transitions that were improperly sized (too small for the tubes). if this is the case with yours, you have two options; one, you can contact Estes and report the problem (and probably get a replacement) or two, glue a wrap of paper inside the larger(payload) tube to make the tubes inside diameter smaller.
Rex