In case you are changing the data-src/data-srcset attributes of already transformed lazyload elements dynamically, you normally also must re-add the lazyload class to the element.
This extension automatically detects changes to your data-* attributes and adds the class for you. This is very comfortable in case you are using highly dynamic or reactive View libraries like React, Angular, Ember etc..
In case you are using React you can also try the following react-lazysizes module as another possible alternative.
// never try to import *.min.js files
import lazySizes from 'lazysizes';
import 'lazysizes/plugins/attrchange/ls.attrchange';