First thing you want to do is to remove the suspended tray (the one you'd take out for doing mugs) and check the hose at both ends for cracks, splits, or holes. If you find any, just cut them off to leave a clean, sharp edge again and re-attach. Put your tray back in and try the suction again. If it's still not pulling the silicon sheet clear then you need to unplug the machine, flip it upside down, use a long screwdriver, and open it up. It's entirely possible that the hoses going into the vacuum pump itself may have become cracked or split so you'd be best to check that first. You can test it while it's still sitting open (be careful not to touch anything electrical inside it, though) by starting the suction up and putting your finger over the end of the hose which attaches to the bottom of the suspended tray. If it sucks properly and the vacuum display goes into the red then you've solved it. If it doesn't, then you need to REALLY open it up and go to the hose which attaches to the underside of the bottom tray.
This takes much longer, and can be a little fiddly. The first time I had a hose issue it was with one of the ones attached to the pump itself, and it was easily fixed by cutting the hose a little but there's not much margin for error on those ones. The second time was with the hose underneath the bottom tray, and that was a real pain to deal with. Cut it and re-attached it though, and all was well. Every other time (which was practically weekly for a while) was with the hose that you can see when you lift up the suspended tray. It would tend to crack where it attaches to the suspended tray, but sometimes it would break at the base instead.
After having to cut the hose far too regularly for my liking, I started buying different hoses to try them out. In the end, I replaced ALL my hoses using the one from the link below. You can't use the screw to hold it in place as it's too thick for it to properly fit, but the hose itself is tight enough that it pushes on and doesn't come off. Now, my entire machine uses this thicker hose and I've not once had to cut it again. Not sure how long it's been, but my guess is that it's at LEAST a year since I've had any hose issues, thanks to this reinforced hose:
http://www.autosiliconehoses.com/3mm...lue-black.html
Hope this helps