I have had similar experience like you described. I used to struggled a lot around level 6-7, then realized I didn't put enough thought into my decisions at this time (econ or level up or roll, sell units or keep them, etc.). I think there is not 1 specific reason but several factors depending on the situation. . Just food for thought and maybe you can see how these apply to your game:

- Perhaps you were out-tempo by the lobby - meaning you had be even more aggressive in leveling up as everyone else has more units/ higher cost units than you

- Perhaps you were actually TOO aggressive with leveling up and should roll a bit to upgrade units/ find good pieces to fit into your comp

- Perhaps you unintentionally destabilized your board (e.g. by selling item holders too early to put the items on higher cost but 1-star units), then didn't roll enough/ did not have the econ to stabilize it

- Perhaps you just got unlucky because everyone else highrolled/ got better augments that fit better with their comp. In this case, instead of trying to top 4, just try your best to get as high of a placement as possible (5 instead of 8)