The reason I asked about carpet is because I was amazed at how much mine compressed when installing the brackets. My deck is aluminum as well and I have had my Spiderlocks mounted for about 6 weeks and have yet to take them off. No loose screws here. Your pilot holes may be too large as G suggested. You could go up a size in screws and that would eliminate that as a possibility. I don't know if the screws that come with them are #10's or #12's. They make a drywall anchor called a butterfly toggle bolt. Those would work but you'd have to cut all 4 screw holes out large enough to get them threw the deck.
It's try larger diameter screws first. Get Stainless steel screws. One word of caution... I use stainless steel screws pretty often and the heads are very easy to strip out. If you could find some screws that have a square head or allen head, it would be much easier to thread them into the holes you have now without redrilling the hole.