PT 12 HD loses sync but only indicates in session window

[Nick Namnone]

I am running Pro Tools 12.4 on a Mac Pro (late 2013). OS 10.10.5. 3.5 GHz 6 core Intel Xeon E5. 32GB 1866MHZ DDR3.
Sync HD as clock source. HD IO 80 analog inputs/outputs. Two HDX cards.
I am having an issue where Pro Tools loses sync but doesn't indicate it on the Sync HD, The IO and there isn't a pop up error warning. The only way to know that sync is lost that I have observed is if the session window is open showing the sync set up and time code offsets. The locked light will not be green. Someone else showed told me a way to resolve it by changing the clock source. I use the Synch HD as the clock source. Their way to reset sync was to start with the last IO and make it the source and set it to internal and then work back through the IO to the Synch HD. I did a search in the DUC and didn't see this but obviously people besides me have run into it. Why am I randomly losing sync and why am I not seeing any error pop up? The front of the IO all show Loop sync.
thanks.