I can easily pin point the exact problem with this anime: It has a premise and nothing ells.
Why? Who? Where? How? 4 things this anime never goes into. Why should I care about these teenagers finding Nanana’s treasure? How did Nanana obtain these magical items to begin with? Who created the magical dungeons that house these treasures? Why is nanana trapped in that room, was she killed in there? How/where do they get the information on the locations of these chests? They ask nanana in some episodes, but for the majority of the time they just randomly know where it is (they are not that hidden if you ask me).
What is the plot of this anime? T-o I think the mane plot is trying to find out who killed Nanana (I presume).
If you want to see some teenagers go on adventures this is a fun watch however, if you want to watch a story with a beginning, middle and end don’t watch this.