Why is my music automatically deleting itself from my iPhone 6s?

Hi

i have an iTunes Match account, where obviously all of my music is stored, and this is fine when I have good 4G/Wifi signal to stream and listen to. But sometimes I need to download music to the phone for listening to offline, as I am a keen runner often running through no signal. But over the past few weeks I have noticed that downloaded tracks are disappearing from my phone (not available offline)

this is very frustrating and pointless as I have plenty of space on the phone, and am having to download music to the phone each time I go for a run!!!!

so, can anyone tell me why this is happening .....and more importantly.....how to stop it!!!!

thanks

Malc