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. 
I agree with Jenn about the syrup, but I usually go around and cut those hard edges off with my handy kitchen scissors. Then I use a liqueur as my syrup. If it smells burnt, I cut off a thin layer of the bad part as evenly as possible. I hate to waste.
I KNOW EXACTLY what you mean when things go wrong. Every darn time for me!!! UGH!! It is exaperating, isn't it?
Yeah...if the warm simple syrup doesn't soften the edges sufficiently, I go ahead & trim them too. But sometimes it does the trick and no one would ever know that the edges were once a little firm...lol. If they do get really hard, then I usually go ahead & trim first and then apply the syrup just to be safe. From what I've figured out, it's just all trial & error. Hopefully that will end when we get our house built & I have a really good commercial style oven!! 
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