What do the instructions of your recipe say? I have made plain jelly rolls (not chocolate), but all the recipes I have ever seen tell you to roll the sponge cake (without any filling), while still fairly warm, with a smooth weave kitchen towel (sprinkled with confectioners' sugar, or in the case of chocolate...cocoa). When cooled off, *very carefully* unroll and put in filling. This is a fairly delicate proceedure, so unroll with an easy, and delicate touch! After filling is spread in, very carefully reroll.

Think rolling in kitchen towel while fairly warm is the key! Don't know if my suggestion will be of any help, but it is worth a try.