Need Masochistic advice for 1. 3betax

I'm in the middle of year 4 on a Gigantic tight clustered map. My influence is about 1/3 the map, Torians already defeated - terrans, drath, and arceans are all but defeated, but are hanging on to 2 or 3 worlds each well within my influence. The influence I am exerting on their planets is (15)+ on each one, but they won't flip. I am massing transports and warships near my front lines of the 3 remaining races to make a sweep of the map within the next 10 turns or so. Should I commit some of my invasion force to go all the way back across the map to take care of these straggling worlds, or can I hope for my influence to flip them? I'd rather not have to take them militarily if I don't have to. I don't have any warships in the area, all my military is near my front lines, it'll take me 4 or 5 turns to get warships capable of breaking their planetary fleets, as they have massed some pretty big guns into their orbits.

Earlier in the game I was flipping planets with only (5) or so influence, but now (20) doesn't even seem to do the trick. Espionage doesn't show any special projects that would prevent them from rebelling. They've had skull and crossbones up for almost 10 turns. I was counting on influence to sway them for me, so I guess what I'm asking is if there is a bug with influence or is it working like it's supposed to? I'm trying to micromanage this game the best I can so anything that saves me a few turns and some resources is a plus.

Thanks