Marks in resin after several days curing

I am relatively new to working with resin and have only used this brand as a top coat for wood pieces and ceramic tile coasters.  My problem is that even after days or weeks of curing, if I stack my pieces they are leaving dimple marks in the resin.....  Is this common?  I saw one video where the girl said she uses wax paper between her coasters.  I am hoping there is a more permanent fix for this.  thanks for any help