Unable to add some items to my watch list

[2007darlow]

A week or so ago, without any logical reason, I suddenly found that when I click on 'Watch this item', some items do not add (ie nothing happens) whereas some items do add normally. This seemed to be entirely random as to which items would add and which would not. When an item will not add, holding down on 'Watch this item' generates a browser window which states 'Watch this item - about:blank#blocked'.
I used 'Live chat' a few days ago and it was suggested the cause could be a browser issue and to delete cache & cookies. I have done this a couple of times - both times then enabled me add one item which previously would not add. Since then I have now realised that items which will not add are those where a choice ( e.g. Colour, size etc) has to be selected. When the listing is just simply one item, I can add without problem. This is totally odd and baffling, not to mention frustrating. Any help / suggestions appreciated please.

Many thanks.