how to force crop for featured images on single page?

Hello Zelatech !
The issue is due to that you were using image size 520 x 337px. We mostly use the size 500×400 for all the article images. The system will automatically resize and get the smaller images for the website. To be specific:
– 500×400 images are used for:

• Widgets:

1. News Slider
2. Featured Image
3. Single post và Featured Image

• First article of Category Archive

– 230×130 are used for:

• Widgets:

1. Recent news, Featured News, Popular News (If you select Medium size images)
2. Category (3 cols)
3. News Carousel

• Articles on  Archive page

– 110×110 are used for:

• Widgets:

1. Recent news, Featured News, Popular News (If you select Thumbnail image size)
2. Category (1 col)

Hope you find this useful

Please log in to Dashboard > Setting > Media, then change the image size:
See the screenshot: