Blanket baggy in centre

[Fraggle]

Hi made a blanket containing one big square surrounded by 4 square on each side , but unfortunately middle is baggy doesn't lay flat is there anything I can do to sort this . First time making ap blanket is there away I can shrink the middle

[Granny Square]

Ugh.  Short answer - no, not without ripping and starting over.

I make a lot of in the round center out things (doilies), which are sensitive to a person's stitch-height tension differences versus the designer's, I make short-ish stitches and I'm very often having to tweak things to get the item to lie flat.

Making thins in the round, like a granny square made center out, you have to obey the laws of geometry, with regard to the diameter and circumference (I know, it's a square, but made center out in the round, circle rules still apply).  'Baggy in the middle' sounds like cupping to me; you can look at it 2 ways:

The distance of the fabric from the center to the point the bagginess seems to end (circumference) is too great--solution is shorter stitches.

There aren't enough stitches to spread out the fabric to keep it flat--solution is add stitches.

Depending on the pattern, you might want to do a combination of the 2, and not a bad idea to track of your changes, you may have to do some more ripping back as you go to keep it flat.  And also because you will be no longer following the pattern, but re-writing it as you go.  Depending on the pattern, it might be easier to tinker with stitch height than stitch count--up to you.

I've had a couple of doilies where because of my opposite problem (ruffling) I've taken really drastic measures - I can think of a couple of doilies where by the outer rounds I had omitted over 100 stitches per round, and another one (all in US DC) I ripped out all but the first couple of rounds and subbed trebles instead of doubles to keep it flat.