If your banana cake is sticky on top, it could be due to several factors. One possible reason is that the cake was not completely cooked through, resulting in a gooey and sticky texture. To avoid this issue, make sure to thoroughly check the doneness of the cake by inserting a toothpick into the center. Another reason could be excess moisture from overripe bananas being added to the batter. Overripe bananas have higher water content, which can contribute to stickiness. It is important to use ripe bananas but not overly mushy ones when preparing banana cake. Lastly, storing the cake in a humid environment or wrapping it while it's still warm can also cause stickiness on the top surface. Ensure that the cake cools down fully before covering or storing it.