Must be putting on a big chunk if 15 seconds isn't enough time to burn it .
Although, many many people have weird style to how they smoke. Are you sure you're not just burning ' ash' ?
You also get the ones who burn the ash 2 or 3 times even though there's nothing left.
( Worst one is when the crack belongs to 1 guy. Naturally he goes first, then he just holds on to the pipe , talking or silent.
And you/ others are sitting there thinking "come on man, pass the effing pipe ".
Hated doing it with others, whether the rock was yours or not. Too many act really weird, carpet surfing , paranoid looking out curtains especially.