I had a heck of a time with AV juices initially due to their viscosity. Normal condom method and syringe did not help.
So, I use a modified condom method combined with patience and it works great. Here's my technique for AV juices:
Fresh carto... I fill the condom about 2/3rds full, then push the carto down slowly (like over a 10-15 second period) looking down the center hole to make sure the juice doesn't just flow up there. My goal is to keep the top completely dry right now.
I do this 2/3rds full/push down two times (this is on the larger Resurrector cartos, YMMV).
Then just fill the condom about 1/4 full or so (maybe 5-8 drops), put the condom on partially (leaving about 1/4" or so hanging the bottom), then gently squeeze the bottom of the condom to force the juice up. Again, I don't let it just flow up the center and flood the top... I keep the bubble forming there just at the top surface by controlling the amount I'm squeezing.
When the condom is dry, refill the condom and repeat this squeezing method until I start seeing the filler on the top start to turn dark from the juice coming up from the bottom of the filler. It's very obvious if the top is still dry and white.
Repeat (maybe with less juice in the condom, as you're getting close to done now), and eventually you'll see juice bubble up from all over the filler and around the sides. I let it sit for another few minutes to soak in well, blow it out into a tissue a bit, and vape.
I top off through the top, but every few days I repeat the partial fill / squeeze method to fill from the bottom again when the flavor tends to get muted.
Hope this helps...