I have found that downloading songs in Spotify (version 184.108.40.2065) onto my Samsung 64GB microSDXC in my Galaxy S5 results in incomplete playback; the songs will stop playing before the end of the track and the transport will not move any further. If I restart the song from an earlier point it will playback instantly. So spotify knows how long the song is, tells me I've downloaded it but won't play the whole song.
Cheers guys a combination of both your suggestions appears to have sorted the problem. A clean install of spotify, followed by a format of the sd card, followed by disabling power optimisation option for the spotify app seems to have sorted it.