Thanks for the question, and sorry to hear about the troubles.
Can you take a look in iTunes at the songs that aren't being seen in the SoundTouch app? We want to check if they are copyright protected. This guide will show you how to check for protected files in iTunes. Anything that is labeled as protected will not play in the SoundTouch music library due to them being locked to iTunes playback by the seller (Apple).
Let me know if you have any clarifying questions regarding this!