Hello rukhsar212,
It sounds like your photos are synced with iCloud Photos and set to optimize your device storage. To ensure the high-resolution version of your photos download, let's check a few things.
To get started, ensure that you're connected to Wi-Fi. Connect to Wi-Fi on your iPhone, iPad, or iPod touch. Your photos should load over a cellular connection too, but let's rule out any data caps or slow speeds.
Next, be sure you have Low Power Mode disabled. Low Power Mode temporarily pauses iCloud Photos. Use Low Power Mode to save battery life on your iPhone
If those steps don't resolve the issue right away, close the app and restart your iPhone. Close an app on your iPhone or iPod touch & Restart your iPhone
Cheers!