Once you are done cooking your dump cake, you definitely need to crack the lid a little bit to allow any steam to be released...otherwise that steam will just be trapped in the Crockpot and the condensation will soften the cake. Sometimes a gooey dump cake can also be caused because you may have used a cake mix that is "made with pudding". Cake mixes that have a pudding base sometimes don't set as well in a slow cooker.