This is a stuffing type carto of 1ml size?
You have to let them sit and not use juice that is too thick to spread through the stuffing
I only buy soft cap cartos and I hate SLBs with a passion (the ones with the fat wad of stuffing and the inner seal - they just suck so bad for me). I buy Boge soft-caps (standard resistance only because I am using slim batteries)
I carefully pop the soft cap out with an unbent paperclip without catching any of the stuffing or wires. I shake my juice and uncap it. I hold a bit of tissue on the threaded end hole and angle the carto. I drip at least 18 drops into a 1ml carto, more if it wants it. Then I do clean up - I make sure the threaded end is clean and dry. I tilt the open end down into a tissue and puff into the threaded end to clear the central air channel (because Boges has center air channel unlike fat stuffing SLoB cartos). I want to see light through that air channel. Then I sop up any juice pooled above the stuffing, cap the carto.
If I was just topping off then it is ready to vape. If it was a fresh fill of a dry blank I lay it down. I use 70pg/30vg juices and for me 15-20 minutes of laying the carto on different sides is enough to get every bit of stuffing wet enough so I don't singe the carto when I start
vaping it. If your juice is thicker than you may need to wait longer. If your juice is extremely thick it may get hung up in tight carto stuffing and never get it all properly damp so thin your juice first.
Then whenever the vapor seems a bit light or a bit dry or the carto is getting too hot, I add a bit of eliqud (first removing it from the battery, holding a bit of tissue on the end if I am adding a good amount). On my main flavor cartos I use a plastic or delrin drip tip as a mouthpiece so that is easy to pull to add juice and also cool on the lips.