Have you tried removing Spotify from your music services in the Sonos app and re-adding it.

Thanks for replying. I have removed Spotify from Music Services and then added it back several times. Once or twice that has corrected the problem for a couple of searches, but then it drops out again, with the exception that the searches do include Spotify playlists (not my playlists). I have no problem searching using the Sonos app on my desktop computer, so there is an iOS issue here. It was working perfectly until last week. I can browse Spotify within the Sonos app and play music from it, so the problem is related to the search function not picking up tabs fro Artist, Album and Song, but recognising Playlist for the search criteria. Irritating and frustrating. 

A quick update on the Spotify search problem. I have said above that Sonos searches are not showing anything from Spotify in the artist, album or songs search results tabs, but Spotify is included in the playlist tab. I was just playing around with a search and in the playlist search results tab I clicked on the Spotify logo in the search list and when I then clicked on the artist, album and song tabs Spotify was magically included. When I did another search I had the same result i.e. Spotify did not appear in the artist/album/song tabs, but when I went into playlists and clicked on the Spotify logo, it was then included in all of the other search tabs. I haven’t a clue what is going on here, but now I can search again at least.

This happens to me too. I've also deleted and reconnected several times in an attempt to resolve. But still persists.