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.

Thanks