I can only put in as many different essentia types as I have cells, one per cell. Even if the tooltip for the cell says, i.e. "135 of 1024 bytes used" and "1 of 12 essentia types stored" it does not accept any type of essentia other than the first one that happened to be stored there. Neither via import buses nor via essentia terminal.

Minecraft 1.7.10
thaumicenergistics-1.1.2.0-RV2.jar
Thaumcraft-1.7.10-4.2.3.5.jar
appliedenergistics2-rv2-stable-10.jar

I dont know if you still support version that old, but if the problem is relatively trivial i.e. some other mod is known to cause this, I could try and fix it myself.