Music keeps disappearing on my iphone 6s plus

I got my iPhone 6s plus i got a few months ago I sync music that i had on my Moto X that ive had forever that ive downloaded and now my music keeps disappearing whenever i try to sync the music im able to listen to it for awhile and then ill close out of it for a few hours and its gone please help me on how to fix this i cant live without my music haha

Sincerely Tasha Hathorn

ps. answer back ASAP please