No lube. Probably whats happening is the studs you are attaching to are not all in line with each other. When you attach one piece it is forcing a slight bow so its not straight and the next board is. Or, your boards are bowed a bit forcing you to bend them to line up properly.
So, what can you do.
Get straight boards and attach them to straight studs. Probably not pracical.
So, you will need to deal with it. Are you hand nailing or using pneumatics?
If using pneumatics you can nail in the first board then use a pry bar to pull it off the studs by about 1/4" at each stud. Attache the next board then pound the face of the previous board to snug it down to the stud. Repeat for each board. By prying up slightly you will have more space to mauver each board into place.
If hand nailing do similar but do not nail the nail home so you have that play. Or do nail it and set the nail, then pry off a little as above.
When I have encountered this it is not usually every board, just a few. I try the regular way to fit in the next board first and only if I need to do I loosen the previous board a bit to make it fit.
One more thought - Take a couple of 12" long scraps. They SHOULD go together fine. If they don't it IS possible that the mill messed up and milled them poorly.
If that's the case then take them back for an exchange. If you cant do that then you may want to consider widening the groove a little. Reply back if you want a good technique for easily making the grove a little wider yet still ensuring its centered.