Question about Arabic vowels?

In Arabic, why do some words have only a long vowel but others have a consonant with a short vowel PLUS a long vowel after it? For instance: ('aadhaan) أذان this only has a long vowel, but (dhaaba) ذاب has a long vowel AND a short vowel. Thanks for any help you might have! I'm desperate, and I'm learning out of a book so I have no teacher :(