I'm looking for a blog article I read once and thought I saved. Basically, the writer said she'd tried the conventional "insert one or two fingers (your own or someone else's) and then repetitively stroke backwards in a 'come here' motion" for a long time and she'd had no luck at all.
But she did feel the spongy tissue that is supposed to indicate the right spot. So she started pressing upwards toward her navel with steady, firm pressure. She said she just approached this with no expectations and no time constraints. Bottom line, this upward push worked for her where the stroking/rubbing didn't.
So, maybe something to try. I also find it very difficult to try this on myself without a curved toy. And mine doesn't vibrate. Not that I've had success, yet, but I still try sometimes.