I am not sure what might be the issue, I have never used, or seen, the Coleman cast iron. Perhaps there is something in the metallurgy that is hindering the seasoning-I know from taking to some folks that enamel coating has issues sometimes depending on surface carbon (?) so maybe there is something odd going on there. Maybe trying using more oil than you think you need for a while and see if that helps.
I've been wondering if there could be something wrong with the metallurgy &/or the casting process. Haven't tried using more oil than I'd think though.