There are a number of reasons why students find it hard to learn vocabulary. Older students sometimes have a bigger challenge than younger ones do, but in general, if students don't use the vocabulary they learn on a continual basis they will forget it. It's therefore important for them to use and repeat new vocabulary often.
There are several fun ways for students to practice vocabulary. Here's a link to a site that lists some fun ways for students to practice newly acquired vocabulary and use it in a meaningful way. http://iteslj.org/Techniques/Koprowski-RecylingVocabulary.html
Remember repetition is the key to learning and retaining new vocabulary.