Steel cased ammo has a bad rep in Mini's, as a rule. My early Mini-30 has seen a good bit of it before brass cased ammo was common, with no trouble. Now, the steel case stuff goes in the AK and the Mini gets brass case.
"Fail to eject" is a pretty specific description. Suspect immediately is the condition of the ejector.
Either way, if it is mechanical, Ruger will make it right. Run quality ammo to reduce variables in future.