Not working for intermediate image sizes

This is a great little plugin I have used for years but it isn’t working properly for me now. Or, at least, not for showing the intermediate image sizes. It seems to work for the standard sizes. What I am not sure of is if this is just me or if others also have the same problem. It could be another plugin I am using causing the problem or it could be some incompatibility with recent version(s) of WordPress. Would love to hear from others or the developer.

FWIW, I have found a fairly inelegant solution to get it working for me. The code is simple and I am happy to share with anyone who might need it, but it would be ideal if the developer decides to update the plugin instead (assuming the problem is with the plugin and not a conflict unique to my situation).