Acrobat not recognising pictures when I try to make it accessible

I'm trying to make a pdf with 50 images screen-reader accessible by adding alt-text. After a fairly successful experiment to understand the process, I tried again properly, but on second and third attempts Acrobat Pro DC isn't picking up the same number of images. So it skips from figure 6 to figure 8 for example, or simply doesn't see figure 1. Is there a way to force it to recognise an image, or add alt-text to that image manually? I've tried using the original PDF where it worked fine first time, and I've tried re-downloading it and starting again, but different images get recognised each time. Many thanks for your advice.