A simple syrup should help...but the syrup should be warm when you brush it on the cake. If you let it cool too much or completely before you apply it to the cake it won't totally soak into it. I do this: if I have syrup on hand already & it's not warm, then I brush it onto the cake a few minutes after I take cake out of the oven. If the cake is already cool & I discover hard edges or just want to add the syrup for flavor/moisture, then I either nuke the syrup in the microwave for a minute or make some fresh. HTH!
BTW - I can make a million cakes just because and they will turn out great...but I do a cake for a customer and something always goes wrong somewhere along the line! lol Murphy's Law, I guess.