Grass is often problematic. I'm not sure why but I think it has to do with it absorbing moisture. I noticed this when a wad of grass I carefully dried in the sun and which was working for a demonstration stopped working when a summer shower went through and and the humidity went up. Over an hour it went from igniting almost too fast at the touch of a hot spark to resiting great showers of sparks even though it had been kept well under cover.

As an aside, hair sometimes works when everything else is damp. In my experience the longer between washes the better it catches and burns.