How do you permanently kill a Transformer? In other words, "kill" a Transformer such that it cannot be "revived."
The complete and utter lack of consistency in death within a story (besides being a Micheal Bay movie) is the common trope of “Plot Armour”
Sometimes referred to as "Script Immunity" or a "Character Shield", Plot Armor is when a main character's life and health are safeguarded by the fact that he's the one person who can't be removed from the story. Therefore, whenever Bob is in a situation where he could be killed (or at the least very seriously injured), he comes out unharmed with no logical, In-Universe explanation.
I will add in this case it doesn’t have to be a main character, just any character that is relevant to the story being told and not just cannon fodder.
Because of this, without falling back on the comics or old cartoon, and sticking solely to the movies canon, there is no explanation as to why some characters die easier than others, besides plot armour.