There are three environments where you can find a secondary stress :
1. Constructions with separable prefixes, such as re-form (as opposed to reform, without secondary stress on the first syllable). Those constructions are generally found with a secondary stress on the separable prefix, even if this generates a succession of two stresses.
2. When the word is derived (to which there are one or more suffixes), the secondary stress will generally be on a syllable bearing stress in the word from which it is derived (primary or secondary). For example : amBAssador --> amBAssaDOrial. However, when this generates a succession of two stresses, the secondary stress is moved to the first syllable, like : dePARtment --> DEpartMENtal.
3. There is a general "rythmical" constraint in English that states that no word may start with two unstressed syllables. So, if a word is not derived, it will generally receive a secondary stress on its first syllable. Like : KANgaROO.
Hope it helps :)