Sounds like you did not heat it long enough. The plastic will start out thin then change to a thick gooey state then to a hot thin liquid state. This is the state it should be when you pour. Once in the mold it only takes 3-4 mins at most to harden enough to remove from mold. Read some of the older posts on pouring basics to get more info like when to add color, glitter etc. If you still have questions feel free to ask. Hope this helps.
Scott