Make sure you are measuring ingredients properly. If you mis-measure the flour or liquid the batter may be thin and the crumbs will sink into it. Also, check your oven temperature by baking a cake mix cake. If the timing falls within the timings on the package your oven is ok, but if not you may need to adjust the oven temperature. If the temperature is off, this will affect the set of the cake, giving the crumb mixture more opportunity to sink in.