When Google+ released earlier this year custom Google+ URL's to the masses, you may have had the option to get your Google.com/+yourbusiness but with the stipulation that you add a few numbers or letters to make it unique. This may have come as a shock to your business name is pretty unique and you thought "there's no way someone else has this name already". Well, that may have been the case.
Recently I ran into the same situation where Google+ was forcing a business page to add a number after the name. Searching Google.com/+mybusinessname it didn't pull up another page, so at least the name without the numbers wasn't taken.
It may be that you don't have enough followers, or you aren't as active, as someone with a very similar name. This could be a variant including plurals and additions in the business name.
Prove to Google that YOUR PAGE has the rights to that name by being active and having more followers than the next business with a similar name.
Good luck getting your Google+ Custom URL to match your branding and business name exactly! If you have any questions, please comment!