"I do not find my phone."
Translation:Ní aimsím m'fhón.
By asking "Why isn't it correct?" rather than "Can you say...?" you gave the impression that you were following some specific rule that would lead you to that conclusion, rather than just being curious. If you thinking you're implementing a rule, it's helpful to indicate what rule you think your following, so that people who see your question don't have to guess at what your reasoning might be.
The written language documents the spoken language, and the unstressed vowels in mo and do naturally disappear when they come before a vowel sound as the two words run into one another without a stop. That doesn't happen with the long stressed vowel in ní - it is pronounced, and therefore you write it.