When you are running  a very hot fire and place quite a lot of weight on top of the grate it may cause it to bend.  During the first fires the steel mesh is at it softest,  as it's used the effect of heating and cooling  will strengthen the steel grate.  

How to fix this?
Roll the grates upside down and run a fire using the same larger timber -  as it heats up it will bend it back. Flip the grates each time you use your Big Pig.  Leaving coals in the bottom of the fire will help the process of strengthening the fire grate.