Hi,
thanks for your suggestion. I checked that I had the latest drivers, and tried different computers but still had the same problem.
I think it turns out that I was taking my project from my computer at home to one at work, and wasn't loading different library folders the right way and had elements which Archicad was having trouble finding (though they looked OK on the drawings). Since I have got the hang of the library a little more things seem to be quite a lot better in regard to the eyedropper and syringe.