What is the cause of wooden flooring gaps?
The flooring was probably mis-milled.
Wood flooring has to be cut, dried, accumulated, fit, nailed, etc. perfectly or you’ll have big gaps.
Often the manufacturers point the finger at installers, when actually it’s their problem.
The way to check is to hold two boards up and push them together. If one slips through then it’s too loose a fit between the tongue and groove. There’s no amount of acclimation or nailing that can fix that.
We’ve had many gymnasiums floors removed and replaced because it doesn’t meet grade. (You can see the manufacturers cry when it’s chainsawed out of there.)
Follow the Western Woods Use Flooring guise (National guide).
Hint: test a zillion boards to make sure all the tongues fit tight in the grooves and doesn’t slip...even if you shake the board while in the groove.