I had a problem where a simple button was reporting as not function when it came to doing its job in the PLC.
When I came to test the button in designer (the button has a tag write on both mouse pressed / mouse released) at some point the button failed to function in designer (after I had toggled the PLC bit manually from RSLogix 500).
I then entered an extra rung on the button - “print 1”
This failed to print to the console
Having experienced this before I clicked on Save project and then both the tag write and the print function both started to work.
I cannot say for sure how this compares with the client experiencing a sudden malfunction since I am within designer doing the clicking but I have seen this happen many times before.
I have Ignition 7.6.2