There is a range of ninjitsu in which you can't gain training on a horse with focus attack and can only gain with shadow jump or by equipping jewlery to be over the minimum skill for death strike and using that. It's been this way since they lowered the minimum skill for focus attack, they know about it but nothings been done.
Not sure what to tell ya with poisoning, I haven't trained it since the last lot of changes but did before that and gained fairly easily up to 80 (which is all ya really need) just poisoning one dagger over and over on the spot, took like 2-3 days from 50. Moving around tends to lose your gain spot.
I'm currently working ninjitsu (again) and sometimes there is a huge wait between gains but then it tends to gain 3x in a row.
Dunno if any of that helps but there it is!