What reason did you give for wanting to return the item?
If you bought from a private seller and you choose the reason 'wrong size/doesn't fit' that is counted as a change of mind and the seller can refuse the return.
Is that what has happened here?
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)