If you notice that you cannot optimize older images with ShortPixel Image Optimizer on your website and you recently changed your hosting, then it is likely that the problem is related to either file permissions and/or ownership, or security measures from your new host.
Sometimes, if you change your hosting provider or server, even if standard permissions are kept intact, the ownership changes and that’s why the files cannot be overwritten. The solution usually is to ask your hosting provider to verify and fix ownership and also verify permission; they need to be 755 for the /wp-content/uploads/ folder and its subfolders.
In addition, you need to make sure that your host is not blocking the connection from or to one of our IPs. Here’s the list: IPs to be whitelisted