"Bob H" wrote

A couple of years ago, I rotavated my existing uneven lawn and then
levelled it off using a mixture of sharp sand and compost before I
re-seeded it.

Since then with heavy rainfalls the ground has gone like a cobbled street,
or it feels that way when I push my lawn mower over it to cut the grass.

What should I do or what is the best way to take out the indentations or
uneveness?

I have tried rolling it both during seeding and afterwards when the grass
had been growing for a month or so, but that dind't make much difference.

Top dress with a mixture of loam and sand using a board to get it level,
unless the hollows are very deep the grass should grow through. If the
hollows are deep then do it in a few goes, actually you will probably have
to have a few goes at it anyway.
Alternatively if the hollows are very deep then using a spade lift up a turf
and sprinkle soil beneath the grass to bring it up level. Keep watered.

--
Regards
Bob Hobden
W.of London. UK