Frontend or backend? The more context you provide, as well as which Joomla/K2 versions you use, the easier it is for me to suggest possible solutions...

Ensure that K2 comments are enabled in relevant K2 category settings. Take note of any K2 categories that are set to inherit the parameters of other categories (you'd have to edit the category settings in these "other" categories in that case).

If you're sure everything is properly configured parameter-wise and you still don't see the comments rendered in the frontend, then it's most likely a K2 template override issue. In other words, you have customized K2 template overrides inside your template's html/com_k2/ folder and these are either stripped off of essential functionality like the comments or are very outdated (it's usually the first case as K2 always strives from backwards compatibility).

A URL to your site would help me easily check if it's an overrides issue.

Can you point to a specific page please? Perhaps any issues you see are related to caching...

Good to know :)