I just bought a 2007 G35 and the trunk would not close. I don't even know how long it was like that before I even noticed...but once I did, I was kind of freaking out. I looked at this forum and saw other ppl having this problem but no solution. At first I thought it may have to do with that internal trunk latch (the one you pull when someone accidentally gets locked in the trunk) or the switch in the glove box that locks the trunk, but I couldn't find anything in manual about it.
I can't take credit for this because I read it somewhere else. But what I guess happened is the part of the trunk that the latch locks to got sunk a little. Like the bolts loosened a little and caused that mechanism to sink just enough that the top latch cannot grab it. I hope that makes sense. What my bf and I did was take out the plugs and plastic from the rim of the bottom of trunk to reach the two bolts holding that mechanism. We loosened the bolts just enough to pull that u-shaped bracket up (it came up just a bit) and re-tightened the two bolts. i was kinda shocked but it worked! The top latch was able to grab the bottom and shut closed.
hope this helps someone