There are various reasons why different popsicles freeze differently. For example, having a very high water content will give you very hard popsicles since water basically just turns into ice when frozen. Fruits with a high water content like watermelon will result in very hard popsicles unless you combine the watermelon with another kind of fruit or add more of something that stops them freezing too hard. Ingredients such as sugar and alcohol would help with that issue. If your popsicles aren't freezing properly give them a little longer. I always make mine a day ahead so they have all night to freeze.