A half decent bull cross should be capable of a Fox come 18 months of age ,and if anything need holding back from doing numbers.
Doing ones and twos,if bred right,then resting up,kept winning,should be within
It's capabilities.
We're talking 18 months not 10 - 11 months thought and this is where the damage can be done,also working whole not fit or sore will sicken quicker than most things.
I certainly wouldn't be giving up at 18 months though.As has been said some are still immature at that age and could be raring to go in 12 months from that age.
Keep trying and give them some more