Greetings,
If the strap is brand new, give it some break-in time to mold to your wrist. this will help with the overall comfort and fit.
As mentioned, a butterfly/double fold clasp will fit better as it is not as long as a single fold and will center better. As far as I know, IWC does not offer one, but many are available in the after market.
Also, if you are trying to center the "Buckle"...DON'T. Center the folding clasp and let the buckle fall naturally on the wrist ( which will be a bit off centered). Trying to center the buckle will only result in the folding clasp to end up at the edge of your wrist contributing to a uncomfortable fit. And as mentioned, a longer strap may be needed.
If nothing works, you are just one of those that can't wear a deployant and worse case scenario is back to a Pin buckle. Whether it's model 3714 or 3904, you have a great looking watch. Enjoy it !
Best,
Andy