Not Writing to Micro SD Card

I cannot get my phone to write to my microSD card except when I reboot the phone. But, I never really do that. However, when I do, O can set the camera to save pics to the card. But, after some unknown amount of device use, it reverts to the device storage instead of the microSD card.
I have to admit that I am using the same 16 gb microSD card I have had since I got my Galaxy S4. Still, the microSD  is not write protected, or even halfway full. And, I can write to it when I have it connected to my Win10 PC. I HATE the thought of buying a new card only to have the same issue.
So, I am baffled. Any thoughts?


Thread starter
... Can you access your card through My Files when this event occurs? ...

I can still play music from the card and view pics on the card and write to it from my PC (when connected). And I can see files on it in my File Manager HD app, even copy files from it. But I cannot write to it from any phone app.
I will clean the contacts this evening and report back. Thanks for the thoughts.