grass not growing evenly?

The turf could be from a different batch, part of your lawn may be more shaded than the other, the underlying soil may be in different patches..like the 'flat' area you refer to may not have rooted properly..turf should be laid on a sand base of 20mm, some patches may not have been laid evenly..The best thing to do would be to apply a lawn feed over the entire lawn, that should even it out

There are many reason why it might. Standing water could have froze killing it or the seed/early grass seedlings. Just take and lay a dense layer of hay/straw in the bare spots and it will grow. When I worked on a state park we would take grass clippings from side fields and just layer it on and it would do the same.

try new grass