Which phrases? Do you mean "I am not understanding"? "I am not understanding" is grammatically correct but not idiomatic. You would normally say "I don't understand". You could also say "I'm not following you", which is almost the same as "I am not understanding" but IS idiomatic.
Understand - like "love" is one of those verbs that is not supposed to be used in progressive form, but we will find this usage in special contexts, Typically, one may say: I don't understand your presentation; can you go over it again?, or I didn't quite understand your proposal this morning... Here is a link that may help: