weak in something
weak on something

He was a little weak on political theory; but his intentions were fine.

But he's weak on the things that will let him use it effectively.

Since I am very weak on celebrities, my wife had agreed to come along and spot them for me.

You can't say we're weak on foreign policy and national defense.

He was a player full of fantasy, but weak on defense.

He was somewhat weak on technical skills, but had great speed.

He remembered that the computer was weak on fine control at long range.

The magnetic field of the Earth is fairly weak on the surface.

They have been weak on the field and at the gate.

That is not a weak but, on the contrary, a strong position.