A V-block is a precision locating device with a 90° V-shaped groove cut into its top, designed specifically to cradle cylindrical work.

When a round bar is laid in the V, it rests against both inclined faces at once, giving two lines of contact that locate the bar's axis firmly and accurately.

A flat job has no curved surface to settle into the V, so it can only touch the top edges of the groove at best and is not seated on the inclined faces.

With so little real contact the flat piece is neither properly located nor securely held, which is why it cannot be gripped as effectively as a round job.

The student's claim is therefore wrong because flat jobs lack proper surface contact in the V-block.

It is not a matter of the flat job being too heavy, since weight is not the issue.

Nor are V-blocks restricted to large jobs, and a correctly used V-block does not inherently damage flat work; the real fault is the missing contact.

Hence, the answer is Flat jobs lack surface contact in the V-block.