Was i taught to play incorrectly? Progress halted.

I am on day 79 so I am still pretty new but I have loved this game so far. Around day 15 or so, I played with a group who told me that powering up my survivors and unlocking all of my survivor spots was the only way to progress properly.

So I unlocked all of the survivor spots, filled them with matching personality types which got me to Power Level 120 about 6 days ago. Since then, I haven't been able to budge my power level beyond 120.

I am constantly battling with obtaining training manuals and trying to find Legendary survivors with the right personality type to update the few remaining purple survivors I have on my teams.

Did I do something incorrectly? I feel like I had to have messed something up to no longer be able to progress in a reasonable way in the last week.

Is it normal to plateau like this when dealing with survivors and power level?

Thanks in advance!