Galaxy S5 and bluetooth earpiece issue

I have Galaxy S5 phone with a bluetooth Sprout earpiece.
After about 3 incoming phone calls the sprout doesn't seem to work correctly, I press the receive button bu I can't hear the caller. I have had the sprout replaced with the same problem reoccuring... I have rebooted the sprout AND the phone but to no avail....after 2-3 calls it does it again.
Could this be an S5 software issue....although the software is up to date.
I might add the phone is bluetoothed to my car as well with no problems at all.
Any ideas please.

Regards
Michael