InDesign CC: Can't Get Alt Text for Images to Work

Hi, 

I am creating an accessible PDF which includes some images with alt text, but when I test them using VoiceOver, nothing happens (i.e. the images are skipped). 

I have designed the PDF in InDesign CC. I have gone through 'Object Export Options' and written alt text for relevant images, and labelled any irrelevant ones as 'artefacts'. 

I export as an Interactive PDF, with these two options ticked: 'Create Tagged PDF' and 'Use Structure for Tab Order'.

In Adobe Acrobat accessibility check, the alt text is marked as 'passed'. When I look at the Content pannel, the images with alt text are appearing as 'Container <Artifacts>' and a series of Paths. When I look at the Tags pannel, the images aren't in the list at all. I have followed these instructions here but doesn't seem to help: https://helpx.adobe.com/acrobat/using/create-verify-pdf-accessibility.html 

To view it, I am opening in Preview and viewing at Single Page view, and VoiceOver reads the text perfectly, but it ignores the images. 

Can anyone help? Thanks so much in advance