Null data receive

This was working fine, from last 3 days image getting null, some received some nulled. this happens with other data too. for test use https.www version of site. thanks.

Url: discudemy. com/all/1

Sitemap:
{"_id":"discalbest","startUrl":["discudemy. com/all/[1-3]"],"selectors":[{"id":"link","parentSelectors":["_root"],"type":"SelectorLink","selector":"a.card-header","multiple":true},{"id":"image_url","parentSelectors":["link"],"type":"SelectorImage","selector":"img.i-amphtml-fill-content","multiple":false},{"id":"button_text","parentSelectors":["link"],"type":"SelectorLink","selector":"a.ui","multiple":false},{"id":"post_title","parentSelectors":["button_text"],"type":"SelectorText","selector":"h1","multiple":false,"regex":""},{"id":"post_content","parentSelectors":["button_text"],"type":"SelectorText","selector":".attached > p:nth-of-type(1)","multiple":false,"regex":""},{"id":"product_url","parentSelectors":["button_text"],"type":"SelectorLink","selector":"a#couponLink","multiple":false},{"id":"product_cat","parentSelectors":["link"],"type":"SelectorText","selector":"a.section","multiple":false,"regex":""}]}