How can I force a Photo Stream sync on the iPhone?

The Settings page says that Photos will automatically by sent to Photo Stream when I am connected to Wi-Fi, however I've waited several minutes and don't see them in iPhoto. I believe they haven't been sent to iCloud yet. Is there any way to force them to be synced to iCloud from the iPhone?

Fastest solution (iPhone restart NOT needed!): I've found that if I open Photos on my iDevice after taking a photo, and then (after the spinning icon goes away) open Photos* on my Mac, the photo appears in my photo stream.

Notes:

This is way easier and faster than rebooting or resetting Wi-Fi.

It's also faster and more likely to work than just killing Photos Agent and restarting Photos.

(Further evidence showing WHY this works: When I open Photos on my iDevice, I see the spinning icon on my iDevice - IFF I've just taken a photo. If I access my photo stream on my Mac‡ without having opened Photos, the new photo is not there.)

*(or iPhoto if you haven't switched to Photos)

‡(such as by clicking Photo Stream under Media in the standard File..Open dialog box in recent versions of MacOS)

Note: Photo Stream only syncs the last 30 days of your photos.

2020 UPDATE: Recently, Photos was changed; when you open it you may need to take an additional step to cause the upload - but if so, you’ll see a prompt about it.

I had this very problem today. I had taken 40 or so photos on iPhone 4, iOS 5.1, and when I got back to WiFi connection, I waited more than an hour for the photos to appear in PhotoStream either on the iPhone or my iPad, and they didn't.

None of the following worked:

• Switching the iPhone to Airplane mode and back
• Killing the Camera and Photos apps
• Turning off Photo Stream in the iCloud bit of the Settings app (this told me it would delete all PhotoStream photos on the iPhone, so I cancelled)

Eventually, after an hour or so, in desparation I forced the iPhone to power-down, and turned it back on.

Today's photos then started appearing straight away on the iPad. In the time it has taken me to type this, most of them have now arrived already.

Echoing this answer:

1. Open up Activity Monitor
2. Search for Photos Agent and quit it.
3. Restart the Photos app.

I found this post after having the same issue - needing to grab an image from Photo Stream that wasn't coming up.

I found that I had wandered outside my WiFi range, and had reverted to a 3G connection. Upon reconnecting to my WiFi, Photo Stream immediately updated (and I mean immediately :)

I checked this by opening my Photo Stream within the Photos app, and taking a screenshot. Within one second my phone started uploading the new image, and within 10 seconds it was live within my Photo Stream.

If you're having long delays check your WiFi connection - This may not be the only reason for a delay, but it helped me in my situation.