First you need to ascertain that any fault is actually with the S/S because there are many conditions that will prevent it from working, which is why you might see the message "Stop/Start Unavailable" - it's a symptom, it doesn't (necessarily) mean there's anything wrong with the S/S itself.
If you don't see that message but it still doesn't work,[FONT="] first and foremost check the condition of the battery. It should be the right type - for my 2013 car that's an EFB (Enhanced Flooded Battery) - and it must be 100% healthy. An older battery can be fully charged and perfectly adequate for normal operation but not good enough for S/S.
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Do you get that "Unavailable" message?
If you switch the S/S off and then on again do you see the message; "Stop&Start Disconnected" or "...Connected"?
After a stall does the car automatically start when you push the clutch in?
Does the engine ever stop and quickly re-start within seconds?