Do you have issues with any other streaming services - eg You Tube, iPlayer? If not, that would tend to rule out the connection. Do you know if this started after an app update?
Sometimes a simple restart of the phone will clear odd problems. If not, go into Settings|Apps then (I think) Storage and look for Force Stop, Clear Cache and Clear Data. Hit all these. Clearing data will log you out of the app and delete any settings, so make sure you know what these are first. If there's anything within the app's own cache that's causing the problem, that should sort it.
I experienced something very similar with Chromecast and iPlayer on my home wifi a few months back. Nothing would sort it and it didn't seem to be the internet connection. I gave it best and it cleared itself after a few weeks - there may have been some app updates within that time.
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