Most of the Time the IC chips will check out GOOD, but when logic is turned on it circuit doesn't work
This is the predicament that makes you tear your hair out.
As chips multiply, so does the complexity. A logic gate has a truth table with 4 rows and 3 columns.
Add another 5 logic gates and soon your truth table has so many rows and columns, it becomes unmanageable.
Does that describe your circuit? The chips can operate properly, but some unexpected combination of states defies the up-until-now flawless logic of your flow chart.
It's like computer programming. The logic gates do exactly what you tell them to do... no more, no less.
You might find you need to draw up a truth table with 20 rows and 20 columns. Then test all possible conditions by switching inputs by hand, before you can track down one little glitch.
You may wish you could have an 8 trace oscilloscope. Or computerized diagnostic octopus.
Etc.
Your ingenuity and persistence will be called upon for a time, before you can track down the glitch.