The rule for when to use "due to" and when to use "because of " can be made simple. "Due to" is an adjectival phrase, which means it modifies noun and pronouns. "Because of" is an adverb and so it modifies other parts of language, including verbs.
Language is a moveable feast of course, and it may not always be straightforward to identify nouns and verbs. Words and phrases change their functions depending on their position in and relation to the sentence they are associated with. So you can have nounal and verbal phrases that include words that don't look like either.
One simple trick is to see if you can replace "due to" with "caused by". If you can, then you are probably ok. If you can't, you need "because of" or something similar.
It's explained in more depth and with more clarity here. There are a couple of examples too.
