Sigma 105mm F2.8 EX DG HSM will not spin to lock in Canon SL2

[testertips]

Hi all,

Purchased a used Sigma 105mm F2.8 EX DG HSM off eBay. Research says that it should fit the Canon SL2. However, the lens will not turn to lock in. It does fit snug into the mount, just doesn't turn. On inspection the lens is super clean and I don't see any damage around the mount area. Thoughts? Want to make sure I'm not missing something before requesting a refund, thanks.