When I trained mine to come when I whistle, I would pick them up and whistle
at them as I handed them a treat, when I used something like ferretvite,
which they consider a treat, i would whistle the whole time they were
licking away at it.  This seemed to wrok very quickly I had them coming to
my wistle in about 2 days.  Yes I am a proud papa.
 
They sometimes need me to whistle a few times, If they have to come a long
distance they get distracted, darn ferret attention span, but they always
come.  You may also consider the possibility that Loki may be deaf.  I don't
know if you have ruled this out already but if he knows treats are available
when you snap your fingers, it should t least get his attention briefly.
[Posted in FML issue 2373]