Is it clear sheet? A diode will not make much impact on clear plastics - this is a known fault with the wavelength of light that diode lasers emit.
You can probably do many, many passes and get --some-- effect. But a CO2 laser is what you want to use for this sort of thing.
You may have a better time using a very very thin metal foil, which diodes can have an impact on, though a higher wattage is ideal.