Hi, this is a common occurence with many brands of pan paints, including Winsor and Newton. It isn't terrible, plastic is a difficult thing to stick to
I usually just wet the bottom of the paint or put a drop or two of water in the bottom of the pan and leave it all to work - that usually sorts it out.
You could try using a drop or two of Gum Arabic in the bottom of the pan, if you have it, this works even better than the water
Once you start using the paints there won't be a problem as Gum Arabic is part of the recipe for watercolour anyway
Enjoy using your new paints