So I've been encountering this problem in a lot of modpacks, but now it even crops up in a modpack that it should work in (as I've seen it played). Using the Thaumometer, you should normally be able to acquire all aspects by scanning anything you come across. The way it works as far as I know is that you can acquire new aspects if you have their 2 component aspects already discovered.
Now, I've been having a bit of a problem with this. First, the Life aspect, which consists of Aqua and Terra aspects. even when I have both of them, I cannot scan anything that has life in it, as it says "you need to study the nature of life" or some such, depending on the modpack.
I managed to fix this particular issue by scanning an ethereal essence spawned in from creative mode, but I've now run into the same problem with the Death aspect (Mortus), and scanning Ethereal Essences doesn't seem to work at all.
I believe there are a bunch more aspects waiting to give this same problem in my game, I just havent been able to confirm them yet.
If anyone knows how to fix this permanently, I would appreciate it!