Acrobat XI Standard reactivation on new pc

I purchased a licensed version of Acrobat XI Standard in 2014. I used the software until I changed my PC last month. When I tried to reactivate the software on my new PC (I had already deactivated the license on the old one), I received this message: "The serial number XXX is already in use on the maximum number of computers allowed. To use this product, you must deactivate another computer within 16 days." When I click on "Retry activation. One of the other computers has already been deactivated," the window loops and nothing happens. I can't use the program like this.