Flat bench press problems

As the title says I have problems with flat bench press. I am not an unfit individual I’ve been working out for a while and have noticed strength and definition changes.

On incline bench I can push 45s comfortably, but as soon as I lay flat on my back I can hardly push 35s.

I have tried adjusting my forms and other things. I don’t know what else to do.

Am I just bad at bench pressing ? Or does this have to do with a muscle that isn’t strong enough?

Any suggestion would be nice.