I can see the margin of one of the veneers. I guess it doesn't go all the way up to the gum line. How this is usually resolved?

If the margin where the veneer and the natural tooth meet is showing,there are a couple of situations that may have occurred. 1) it was there and you didn’t notice, as you suggest, 2) you had a bit of inflammation in the gum during the temporary phase that resolved and a margin that was covered by gum tissue is now visible. I think if you are thrilled with your veneers talk to the dentist that did them ,the only solution is to redo the single veneer which involves preparing the tooth higher up to meet the gumline.

The space is not an issue unless it is visible when you smile. The only way to fix this is to redo the veneer and make it longer so that is goes all the way to the gum line.

This space between the veneer and the gum-line is called an open margin and it's not normal.  Custom made veneers should line up exactly with the gum-line and should not show an open margin even on a post-delivery x-ray.  A good dentist knows that even if this open margin is not visible to you , it can cause various future problems including food/bacteria getting trapped in the area and causing decay of the underlying tooth.  The dentist should have not delivered the veneer if it didn't fit and should have sent the veneer back to the lab for modification.  They should also take a post-delivery x-ray to ensure that there are no open margins on your veneers.  I would recommend going back to your dentist and having them correct this immediately.  They will need to remove the permanent veneer and take a new impression to be sent to the lab, so they can fabricate a new veneer.  Good luck!!

If you have a gummy or high smile and the margin is visible when you smile, your dentist can usually do some bonding using the same color as your veneer to mask the margin.  If you don't have a high smile, and there is no gap between the margin and the tooth, ignore it