The most common reason for a MacBook to slide forward on HiRise is due to the stand being placed upside down on the desk. The flat plate should be on the desk, while the v-shaped arms should be up, holding the MacBook.
The other reason a MacBook may slide forward is if the back of the MacBook or the soft silicone on the HiRise arms has dust or a film build-up (such as furniture polish, hairspray, cooking spray, or hand oils or lotions), causing the silicone to lose its ability to hold. In these cases, wiping the silicone and back of the MacBook gently with a damp cloth to remove the dust or film should restore the grip.
The other reason a MacBook may slide forward is if the back of the MacBook or the soft silicone on the HiRise arms has dust or a film build-up (such as furniture polish, hairspray, cooking spray, or hand oils or lotions), causing the silicone to lose its ability to hold. In these cases, wiping the silicone and back of the MacBook gently with a damp cloth to remove the dust or film should restore the grip.