Google shopping results do not return the thumbnails. One of our users has sent an example to inspect:
I have recreated this exact search with same parameters but API brings the results with thumbnails this time:
Then our user sent another example:
and reported:
> I have noticed it happens frequently when the very first search in the list of ‘shopping_results’ has extra html in the title: .A2sOrd{font-family:'Google Sans','GoogleSans-Local',Roboto,Arial,Sans-Serif;font-size:16px;font-weight:normal;line-height:24px;margin:0}Brand New No Tags Reformation Gown | Color: White | Size: 6 | Allakapel's Closet
Maybe this can help you with debugging?
Then Ilya commented:
> Found a reason - Google changed JS code that displays thumbnails. Old product pages use var ii=[' 95399590374638197453')]; setImagesSrc(ii,s). Now ones use var _i='4327465023539296548'; setImagesSrc([ i], u). Our code don't match new thumbnails.