Well there are many that don't lose it.
It only happens in words that have two syllables whose second vowel is ı - i - u or ü, like: oğul, resim, hüzün, karın - which all become oğlum, resmim, hüznüm, karnım in possessive first person for example. But not all such words undergo this change. For example: Kılıç (sword) is kılıcım and not "kılcım".
But at least now you know that if you see a random word like "sedef" for example, you will automatically know that it won't lose any vowels, because its second vowel is not "ı, i, u" or "ü". So there you go.