Herb - Mine does the same thing. Sometimes it opens just fine, other times it doesn't. I haven't found anything holding it, and just said the hell with it one day and stuck a screwdriver under the lip and pried it open. It made a louder then normal snap and opened right up. The problem is still there on occasion, but I live with it as I rarely use the thing. Twas me, I'd just use a screwdriver, with a rag against the van body to protect the paint, and open it up. Do it slowly as it's all plastic. Sometimes it helps to push vertically down on the whole cap while opening as it seems to take some pressure off what must be the latch. Once you get it open, you can see where there is a little lip that acts as a catch. You could shave some of it off and that should help.
Good luck, and if it breaks, this message will self destruct and I'll deny ever writing it.
You can be anything you want on the Internet,
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2007 RB50, 6.0