If you are using blocks that you received in a swap, measure them to see what size they are.
Actually, measure all blocks to see if they are the expected size.
If one has changed rulers, or presser feet, or sewing machine, or taken a leave from the project, check to see if the "new" blocks are the same size as the "old" blocks.
One can ease a "little" and it will lay flat. After that one has "gathers."
And the smaller the block, the less wiggle room.