This issue happens often when setting up a new iPhone. Here are some easy steps you can follow to get your messages and pictures syncing properly:
First, double-check that messages are enabled in iCloud. Go to Settings, tap your name (Apple ID) at the top, select iCloud, and then make sure Messages is turned ON. If it's already on, switch it OFF, wait about 30 seconds, and then turn it back ON. This can trigger the syncing process to restart.
Next, make sure your iPhone is connected to Wi-Fi and plugged into power or has at least 50% battery. Syncing your messages, especially ones with pictures, can take a lot of power and data.
Also, verify that your iPhone and iCloud have enough storage space. You can check this under Settings > General > iPhone Storage and also under Settings > [Your Name] > iCloud.
After confirming these settings, try restarting your phone. Hold the side button and either volume button (depending on your model), slide to power off, wait a few seconds, and turn it back on. Restarting helps fix small glitches.
Additionally, check to see if your software is up-to-date by visiting Settings > General > Software Update. Apple often fixes syncing issues in software updates.
Another useful step is to confirm your messages aren't set to delete after a certain period. Go to Settings > Messages > Keep Messages and set it to Forever.
Keep in mind that syncing many messages and pictures can take time—sometimes hours. Leaving your phone connected to Wi-Fi and charging overnight may help the sync finish.
f none of these steps fix the problem, you might need to sign out of your Apple ID completely and then sign back in. To do this, go to Settings > [Your Name] > Sign Out. This will prompt you to enter your password to turn off "Find My iPhone." Once you're signed out, sign back in. This usually solves tougher syncing issues.
Important note: Before signing out, ensure that your important data is backed up in iCloud to avoid losing any information.
If these steps don't help, contacting Apple Support directly might be the best next step.