The performance of an underlay will depend on its density rather than its thickness. A lightweight, low density underlay will depress more than a heavier, denser one, and the thicker the underlay is, the further down the indentations will go. A 12 mm underlay can feel wonderful underfoot, but may not be the most practical in use for the reason you have discovered.

Gripper is usually about 6.5mm thick, so a deeper underlay is likely to cause a downward ‘slope’ around the perimeter of a room, and may make the fitting more difficult as the underlay will tend to want to pull the carpet off the gripper pins, especially if the gripper is fitted with too wide a gap (the British Standard states it should be two thirds of the thickness of the carpet which is far less than many fitters realise).

A thinner, denser underlay would improve things, but it will not reverse the damage that has already happened.
Richard Renouf, founder