arson progress not moving much

I did an arson crime, which asked for full destruction, and it was at 96% and not moving, I then threw one more gasoline onto it and it only increased to 97%

 

fire was raging, too, the flame symbol was pulsing a lot (which I assume means a big flame)

 

but it incremented the burn progress by 1% before taking a nap, which made me fail the crime

 

is this a bug, or am I missing something crucial to the arson crime

Yeah you're missing how it works - this behaviour is quite normal.

I'm surprised you've reached skill level 49 without already experiencing this though?

 

For a 100% destruction all areas of the building need to be at 100% when the progress counter can reach that high - and the highest flame graphic is for the highest range of damage covering the last percentage points - not just "it is 100% on fire"

Depending on how many flame icons there are (each one being a different location in the building) will affect how easy it is to get all of the building to 100%, and counter-intuitively the "centre flame" still needs to be at 100% to the very end even though that was the first part you were burning down so would expect to have been fully destroyed already.

 

Before you light the fire, any fuels you add will be spread around the building more (a bonus to give extra coverage) but after the fire has been lit the fuel doesn't spread quite as far but increases the intensity/growth speed (a bonus to fire strength) - solid fuels do not cover much of an area but keep it intense and burning hot for longer. gaseious fuels cover a lot of area, allowing fire to reach areas which it couldn't previously reach, but don't add much to the "long term burn" strength. Liquid fuels are in the middle.

 

For full destruction you need 100% coverage AND the fire in all zones to also be at 100% strength.

 

If you use scripts, the arson flames prefetch visualizer script will give you a graphical representation of what is happening, burn a few with that running and you will see how it all joins together :)

Arson Flames Prefetch Visualizer