If you are unable to invite a creator to your campaign, it is typically because your campaign is full. Depending on which pricing package you have purchased, you can have 10, 20, or 40 active influencers on your campaigns - once the limit is reached, you cannot invite more.
Tip: If your campaign is full, you can copy it to a new one by opening your campaign, clicking on "Actions" in the upper right corner, so that you can get more active creators to promote for you.
Another reason may be that the creator has already been rejected. You can check this by opening your campaign, clicking on "Requests" and then seeing who is in the "Declined" category.
