Apologies all if this fix has already been described in another post but I did read every post I could find on the WWW and all fixes suggested did not work for me.
If you are still having trouble with the ST app finding your Hue bridge and at the point of throwing one or the other into the proverbial pile of Piece of Sxxx Technology (POST) the try one more thing.
I have numerous dumb switches on my home network with approximately 20 hardwired devices and 5 wireless. I had my Hue bridge plugged into one dumb switch and the ST hub into another. The ST app could not find the Hue bridge after trying all other fix suggestions I found on the WWW. So as a last resort I simply put both on the same dumb switch and magically the app found it this time!
This may be something you want to try first and save time (and your sanity) if you have your ST hub and Hue bridge on different switches!