Laser machine got slower

I was using the same speed and the same power, but then it slows down a lot, but i dont know the reason, what reason can it be?
I was using my laser cut machine to cut the polywood in 3mm and with the suggested speed power and pass, but then the machine just no more able to cut it through as from the beginning
for my laser the suggested speed is 350, 100 power with 1 pass.