in the case of working spaniels i do feel it's because the scent of a pheasant or other game bird is far more enticing to them than a rabbit or a fox. A lot of that is instinctual, but the gundog training focuses it....and a lot of gundogs don't make the grade.
I know if you presented jarvis with a rabbit scent or a pheasant scent he would go for the pheasant any time, but his pheasant keen-ness at present makes him less steady than he should be so that needs to be worked on
Working cockers can be really obsessive - I see one on our walks who is totally ball obsessed and completely ignores everyone and everything except a tennis ball. That's her 'button' (and to be honest it irritates her owner totally as she won't focus on anything else...).
Maybe you have a laid back, middle of the road boy....and there's nowt wrong with that!
I know lots of 'failed' working collies so their behaviour cannot always be 'programmed' either.
(and what is your trainer measuring Molo's 'promise' to? That of a collie?)