Very Heavy Fighter power issue

[Its Raisu]

Off the bat, i can say this is from Tech Compatibility.

But to be able to answer why your getting power core reductions, it is a matter of having more detail pertaining to the equipment on the character:

What ID do you have equipped?
What are the exact guns you have equipped?


(I cant remember if engine is included in techcompatibility)

Edit: Also the link Skorak posted can help identify what equipment combinations will trigger reductions.