Yup, I've had this same issue..... I bought a used Samsung S3 to use on Lucky, and the previous owner said it was locked to Koodo, so I called Koodo, and got the unlock code, which, unfortunately, didn't work. I called them back, and told them the code didn't work, so they asked for the IMEI again, and confirmed that the code I was previously given was correct. I tried it again, and still no workie, so the person I was talking to said they'd escalate the issue, and call me back.
While waiting for them to call back, I did some looking on the 'net, and found some info that did the trick..... I simply Googled something like, "Samsung S3 doesn't accept unlock code", and several pages came up with info about using the "Debugging Mode", and that worked.
I suggest you do a similar Google search, for your particular phone model, and see if that'll get it sorted out.
Good luck
, and let us know how it goes.
And BTW, Koodo never did call me back......
Did some lookin' around, and found this page, that
MIGHT help with the problem:
https://soporte.doctorsim.com/hc/en-us/articles/216722647-LG-Unlocking-Troubleshooting-Guide