A couple of things I have found helpful:
when burnishing, it doesn’t take a lot of pressure. On some of my early circuits I would try to scrub really hard and I found it would take off too much material and damage traces. This then made it harder when trying to solder to it (you want as smooth a surface as possible). If you have an inspection scope, break off a small piece of the burnishing pad and hold it with tweezers and burnish a pad or two while examining it under the scope, and you will see what I mean.
Flux, Flux, FLUX! I cover my entire circuit in flux right after I burnish it. I use an MGChemicals flux pen for easy no-mess application. Even when solder pasting, I have found the flux can help. Also, pre tin as much as you can before applying it, and at least put more flux on all the pads/wires/contacts.
Temperature: I have found this to be absolutely critical. With the new tin bismuth solder, I set my soldering iron to 210 C. I have found this to be the optimum temperature for my equipment, but you may need to try a few temperatures to find the best on yours. Of note, 205 and 215 DO NOT WORK WELL on mine…so temperature is critical.
Put a blob of solder on your tip, flux the area you want to apply it to, then gently touch it. I have also found sometimes if you gently rub the tip on the surface (GENTLY or you will damage it), it helps it wet better.
Hope this helps.
one last thing, just to confirm this is for the Tin Bismuth Silver solder. I use some I got from digikey: Chipquik SMDSWLTLFP32 which is Sn42/Bi57/Ag1, but I believe this is the same alloy that voltera now sells.