Roof A/C covers need to be removed to service the A/C unit. I find a large percentage of the covers damaged because the screws were tightened too much when the cover was re-installed. The screws can easily be tightened to the point that they crack the cover around the screw hole. The screw only needs to be tightened enough to hold the cover in place.
I suggest putting a washer between the screw and the plastic cover. The washer will spread the contact point over a wider surface area and will help to minimize the possibility of cracking the cover. Once the cover is cracked, the vibration of the A/C unit and wind, while traveling, will result in further damage to the cover. If the crack becomes large enough, the cover can detach and fly off the roof during travel.
Inspect the cover for cracks. If they exist, repair them or add a large surface washer to provide good contact between the washer base and the A/C cover. In many cases, using the washer on a cracked cover will provide enough contact with the cover to hold it securely on the unit and minimize further cracking of the cover.