Im on Xbox 360 and i had this same issue. I had purchased them years ago under a different Gamertag than I use today on a different console than I have today. So what I had to do was:
1) Download the Gamertag I purchased them under. Thankfully I remembered. Then I had to go under purchase history and re-download them all.
2) I then signed in to my old Gamertag and went to Console Settings and did Transfer License. This allowed me to transfer the uni's to my new 360.
3) The next issue was that when I copied the uniforms to my new 360 to be used under my new Gamertag, it would show as if I never bought them. Of course when you click to purchase you can't do that anymore so...
4) I had to copy all the uniform packs to a USB...move my old Gamertag Profile to the USB as well as my current Gamertag profile. I did this so the uniforms would show assigned to my old gamertag..
5) Once in Horizon, I pulled up each pack one by one, copied and pasted my current gamertag Profile ID and reassigned the uni's to that profile and it worked.
I dont know if thats the issue you are having but I was dead in the water without Horizon.