Nouman Ali Khan explained in his tafseer lessons of Juzz 'Amma that when Allah says;
...يأَيُّهَا الَّذِينَ ءَامَنُواْ
Ya ayuhaladheena 'aamanu [Oh you who believe] - this is referring to the Muslims in general.
But when He refers to a people as Mu'minoon, then they are of a higher rank than the Muslims (i.e. they have gone through trials and remained steadfast in emaan [belief] - so they have the title of Mu'min).
He continues that Allah does criticize those who claim to have emaan - aladheena 'aamanu (i.e. Ya ayuhaladheena 'aamanu maa lakum? [Oh you who believe, what is wrong with you? - then He will mention a mistake or sin they did])
But Allah never criticizes the Mu'minoon. A level higher than the Mu'minoon are the Muhsinoon/Muttaqun [which is the highest rank - these people are always remembering Allah].