You are either going to have to use a lot of TIM or preferably reapply thermal tape.

Unless there is something terrible wrong the best thing a person can do is not disturb the original interface material on a digital MOSFET. FET to FET height is never identical because of on-board placement tolerances. The die size of these MOSFETs make it difficult to shed the heat that is produced from the loading that we subject them to. And finally because of the mounting method of the heatsink the board has a tendency to bow at the middle FETs. When combined, these issues make conventional TIM a rather poor choice for an upgraded cooling solution.