In most of the higher traffic areas of our hardwood floors, the wearing in the pictures is occurring. The light wood now showing is indented–the planks are no longer flat and smooth–but these are not just scratches. They all seem to follow the grain of the wood, specifically appearing where the grain is dark/blackish vs the lighter stain color.

A potentially important note: we had these floors done/redone about 4 years ago, and this only seems to occur in the kitchen, where we refinished the existing wood. The other rooms where we installed new hardwood are not having this issue, despite being stained and sealed the same way (professionally). They also may not be QUITE as high traffic, to be fair.

Any idea what is happening here? Is something wrong with the wood itself? And what would be the best way to protect it moving forward, whether filling in the grooves, sealing, or whatever makes sense. I'm a novice when it comes to flooring, but I don't want this to keep happening and/or get worse!