Bliss is a lot to ask of mindfulness. If living in the moment put us in a state of eternal bliss, there would be no need to talk about mindfulness. Everybody would already be doing it. Practicing mindfulness will not make everything blissful, it will ensure that you experience bliss when it occurs. Bliss is much more subtle than it sounds. It is there beneath all of the clatter of normal thinking. In order to get there, you have to experience your normal thinking. By focusing on what you are doing in any moment, you sometimes forget to think about something else. That neutral feeling, feels good compared to feeling worried, sad, or bored.
If you are trying to distract yourself from your feelings, you will only notice them when they come back. That is disheartening. If you pay attention to your undesirable feelings, you will notice them when they go away. That is encouraging.
If you are practicing mindfulness and are still not feeling bliss, and you have somehow come to expect bliss, then you will feel frustration. Feeling blissful is not what mindfulness is about. It is just one of the feelings that may occur.