Some images not getting yellow focus when using screen reader

Hi,

We keep having a problem with the yellow focus. Some icons do get the yellow border and others don't, even though the screen reader reads out the Alt text in both cases. All icons are png files and there doesn't seem to be any difference between the ones that get the yellow focus and the ones that don't. Even identical icons receive focus on some slides and don't receive focus on other slides.

Have you by any chance come across this issue before and can you advise how to fix it?

Thanks,

Sabine