Hi there deidre_qld,
Thanks for contacting the Apple Support Communities today. We understand you're having issues with the photos on your iPhone showing in a lower resolution, and we'd be happy to look into what's going on.
To get started, in Settings > [Your Name] > iCloud > Photos, is 'Download and Keep Originals' enabled here or is 'Optimize iPhone Storage'? If wanting to save all photos to iCloud and your iPhone, let's enable 'Download and Keep Originals' here. Check out this link for more details: Download iCloud photos and videos - Apple Support
Let us know if that helps.
Have a great day!