Have you undone all 4 bolts? (If not, there's 2 under the rubber stoppers that stop the boot slamming onto the body).

Otherwise, make sure you're lifting it up evenly (if you're just going off one side it may cause the other to dig in a bit), and check for sikaflex/similar between the spoiler and the boot. There might also be 2 plastic lugs closer to the window that could be holding it down, but I don't think they should give you as much grief as you're describing.