Are the posts set in cement? Is the house new construction where the ground may still be settling because it wasn’t fully compacted?
People often skip cement footers for fence posts which might be okay if the soil is well compacted and the type of fence isn’t sensitive to movement but the one place you should always use cement footers is around gates.
If you already have footers then my suggestion would be to set them deeper. It could be poorly compacted soil or the type of soil that’s the problem in which case the only way to combat it is to dig down deeper so the footer reaches more stable soil.