Isomalt Fail

Hi there. I bought Squires Kitchen Isomalt Nibs for the first time last week. I wanted something special to go on an ‘underwater’ themed cake I had been working on. I followed the instructions to the letter, pouring the melted Isomalt carefully into the moulds and the results were perfect once set. I attached them onto the sugarpaste covering and they looked amazing. However the next day when I checked the cake the Isomalt appeared to have melted and run, leaving just a sticky residue running down the sides of the cake. I rescued the cake and replaced the Isomalt decorations with sugarpaste instead so all is well there but my question is, what did I do wrong? Has anyone else had tis problem and if so how did you resolve it.