I installed/sanded/finished a floor about 2 years ago for a customer, apx 800 sq/ft. They came to me last winter concerned about a large crack that had developed down the center of the house. In the summer, it closed back up and no problem. Well this winter it has happened again, this time maybe even worse. This was the second house I have had that problem with, the other just happens to be next door. My question is why is it only happening down the center of the house, right across the center joist?

Got pictures? I like seeing those cus I rarely do naildown round these parts. So it's kinda scary

Are you changing direction ( using slip tongue ) , at that location, on both houses ?

No changing of direction, just a continuous run from start to finish. I'll go get pics this weekend and get back to you.

the house is settleing seen it a hundred times.

That's the first thing I thought of. But 2 houses at the same location. Well stranger things have happened. Yea, they are next door to eachother
Maybe grading, earth movement, & or compaction issues ??

The fact that it appears in the winter, dry heating season, and goes away in the summer humid seasons, tells me it is related to gain and loss of moisture.

A subfloor seam can cause exaggerate panelizing.

It has been determined to be structural. The ground under the house is full of water and is sinking in the soft fill dirt. I don't know what the homeowner and builder are going to do, but I"ll be there to fix the aftermath and receive a pay check!