The sub-floor is laid on top of the floor joists. It is made up of 3/4 inch tongue-in-groove press board or plywood. Yes, I did say "tongue-in-groove". The tongue is a half-inch WOODEN projection that is pushed into an indentation on the next board. This arrangement is meant to prevent bumps from forming in the floor. It is not an especially pleasant experience for the tongue. I do not suppose the groove finds it comfortable either.
Not surprisingly, the tongue can be resistant to entering an adjacent groove. Imagine what it would take to get you to put your tongue into a groove, even a plywood groove. To encourage it, you will need to apply my favorite motto: "If it don't fit, force it."
Find a sledge hammer. Place a 2X4 next to the plywood to be struck. To hit the subfloor directly can cause enough damage to make it impossible to fit the next tongue into the groove. You must be firm but gentle. Pound on the board until you have a union between the two boards and nail them in place.
As disgusting as this procedure sounds, it makes an excellent platform for the next stage of construction.