Greetings mikaela290,
Thanks for visiting the Apple Support Communities! I understand you don't see all of your photos on your iPhone as expected. However, you mentioned you saw all of your photos when you visited iCloud.com.
The following support article provides helpful information for using iCloud Photos: Set up and use iCloud Photos. Within the guide, you'll see how to ensure iCloud Photos is enabled on your iPhone. The guide also mentions that depending on your internet speed, the time it takes for you to see your photos and videos on your devices might vary.
If the issue persists, I suggest ensuring the iOS software is updated for your iPhone. If an update is needed, let's have you create a backup with iCloud or iTunes first. You can see those steps from the following guide: Update your iPhone, iPad, or iPod touch
Additionally, I suggest testing the behavior while your iPhone is charging.
If the issue continues, let's have you ensure Low Power Mode is off, and test the behavior. You can see that setting from the following guide: Use Low Power Mode to save battery on your iPhone
Further, let's ensure you have enough local storage on your iPhone for the photos to download: How to check the storage on your iPhone, iPad, and iPod touch
Let me know how it goes from here.
Cheers.