It’s likely because you are using a diode laser with white paper. Because diode lasers use light in the visible spectrum anything that reflects that light won’t work well. White paper is a prime example of this - it will initially reflect most of the light but as it heats up it’ll start to char a tiny bit which eventually helps things along. But move to a new location and you have to basically start all over again.
Cardboard is a much better option for doing tests like this. I’ve also had a lot of luck with cheap construction paper.