did you by chance press the item after marking it? heat setting can cause the chalk color to (set)
anyway- the one i had the same problem with (pink chalk) came out when i finally simply washed the item- not soaking it- not rubbing it- but i put it into the washer with other items- and washed it-with detergent and a normal cycle- checked it when it came out and sure enough it was all gone.
then i dried it.
you can contact Clover- their web site has tips on removing the chalk-they offer help- and if you don't find help you can call customer service and they will talk to you (although today being cyber monday it may be hard to get through on the phone. i recommend visiting the web site. and for future projects- be sure to not set the chalk, mark as lightly as possible and still be seen, and wash the item when finished-sometimes i'm afraid people set there markings when they are trying so hard to remove them- but don't just wash them out. i have a friend who has had good luck using Magic Eraser's to remove markings. good luck