Dimples, or sink-marks, usually appear where there is a change of section on the reverse side of the component, such as ribs, bushes or other changes in geometry.

They are caused in the material shrinking more where it is thickest.

Reducing the speed of the cure, by working at a slightly lower temperature will help.

Using fillers will also help reduce sink marks.

If there is no change of section by the sink mark, it is probably caused by the release agent.

If using a silicone tool, try using no release agent.