Janet Baraclough wrote:

You can make a cross-shaped slit in the turf, peel it back, either
excavate or add some soil (to lower or raise the level) and put the
flaps back down on top.
For hollows, it's much easier just to gradually add fine soil on top
and let the grass grow through it.

I had the same problem. What I did was to fill up the holes by
spreading (raking) loose top soil over the affected areas and reseeded
where it was needed.