I casted a fair bit so this bugs! I got a new aluminum 6 cavity mold for 9mm. I put black ash/smoke via acetylene torch. This has always worked well. Then it started to get sticky, so I cleaned it with a soft rag only and then tried paper smoke. Worked for a while. Now one cavity drops right out, three with a soft tap, one with a coupled of taps and that darn forth cavity just does not want to play well with others. Why? Seasoning tip? Mix of metals? Oh yeah, this is WW and tin form solder. 1:10 solder to WW ratio.